A flare is quite simply a return of the PMR symptoms after a period where you have NOT experienced them. It may be because of reducing too far so that the dose of pred is no longer enough to manage the level of inflammation due to the activity of the autoimmune disorder underlying it all. Or you may have remained at the same dose but the activity of the autoimmune part has increased and there is more inflammation. Or you might have managed to get off pred altogether at a time when the autoimmune part was quiescent and then it starts up again.
Like the others - I am somewhat surprised you were diagnosed as having PMR when you had no pain - it is usually one of the criteria. However, be that as it may - in response to your question "Ive been told to reduce prednisolone by one daily mg every week, (this week on 5mg per day) . How lousy should I expect to feel on that?" the answer I'd give is "Pretty lousy and your doctor is playing with fire".
Why did your doctor tell you to reduce at all? Has s/he decided it isn't PMR? And for goodness sake - why so fast?
At least two things are probably at play here. The first is that reducing at that rate is very very likely to lead to steroid withdrawal symptoms as the body is suddenly asked to adjust to a lower dose, if you remained at the new dose for a few weeks that feeling would improve and then you could start on the next step. However, you are then asking it to do it all over again before it knows where it is. Add to that that the recommended max reduction is not more than 10% of your current dose - 6 to 5mg is 16% and the percentage will be greater with each mg.
The second, probably more important, factor is that if you have been on pred for more than a few weeks, once you get below about 7 or 8mg your body has to start producing its own corticosteroid, cortisol, again. It doesn't just manage that perfectly overnight and in some people it takes longer than others. It is less the adrenal glands that make the stuff being able to do the job, it is the very complex feedback system that involves several glands (the hypothalamus, pituitary and adrenals, the HPA axis) and which swings about like a pendulum that has been disturbed until it gets to the right position again. Even if your body achieved it in a week - asking it to do it every week is far too much and I'm not surprised you feel lousy.
If your body doesn't have enough cortisol or other corticosteroid it cannot function properly - and you are possibly heading for what is called an Addisonian crisis if you continue reducing at this rate.. This leaflet is about that - you don't have the problem because your body is unable to produce cortisol and you have some available as you are still taking 5mg, nearly what you need for basic processes but it can't cope with more than that. You feel so lousy because your body is begging for you to slow down so it can cope. You aren't likely to develop a full-blown crisis as long as you don't reduce any further for the moment. However - do the symptoms sound like you?
https://patient.info/health/addisons-disease-leaflet
You really need to go back to the last dose at which you felt well - probably 7mg? Then you need to reduce MUCH more slowly, the rate recommended by most doctors from 10mg is 1mg per MONTH but we know that even that is too much from one day to the next for many patients and they suffer a lot of discomfort because of steroid withdrawal. Half a mg every 2 weeks is far better - and we recommend a "Dead slow and nearly stop" approach which spreads the 1mg change over a month so the body isn't having to adjust so quickly.
However, we aren't doctors and you'll have to get a doctor on side. It's the weekend - stop reducing now, see at least your GP as soon as possible and ask them to please rethink this approach. If you feel any worse than you do at present call the out of hours number and ask for advice.
Now we'll get to the blood test: ask your GP to arrange an adrenal function test, also known as a synacthen test. In case they argue you need to stop pred first, no you don't, it can be done while you are still taking pred, it just has to be interpreted differently. It will show if your adrenal glands are still able to function well enough - and that means you will get back to normal by reducing more slowly. There is always the chance that it isn't going to function however slowly you go about it and you will need to stay on a very low dose as a replacement instead and that must be discounted first before reducing any further. Endocrinologists have said they wish all patients who've been on pred for more than a couple of months were tested as it would save a lot of trouble.