5 day periods have turned into 3 day periods

I am a 20 year old girl and my periods have been very regular for 4 year always exactly every 4 weeks and always lasted 5 days. Usually would start off light on day one become heavier on days 2 and 3 then slow down again. However for the last 2 months my period has been 3 days long and very light, on day 3 it turns into brown discharge. I also used to get very mild cramps on the day before and the first day of my period and my boobs used to hurt which would be a massive indicator that my period was about to begin. However for the last 2 times I have had no symptoms leading up to my period and have had really bad cramps to the point where i could cry just on the first day of my period. The first time this happened I took a pregnancy test which came back negative. I don't know what it could be? Has anyone else experienced this? Thanks

Did you lose any weight recently? If you lose too much (more than your body is OK with) your female hormones will be reduced and you can experience (among many other things) lighter periods, not ovulating (so not releasing an egg each month and so being infertile) and shorter periods. If you are concerned, your best seeing your GP to check things out. Did you see your GP before posting and if you did what did they advise?

I haven’t lost any weight recently, not even been trying to. I haven’t been to see my GP yet, but am due to come on within the next week so if it happens again i will book an appointment to see if they can find out whats going on

Hopefully it was a one off issue- don’t forget, there are multiple issues that happen in our lives that can affect our cycle.
“Simple” things like stress (if stress is ever simple…!)
Lack of sleep,
Even if we’ve had a drastic change to what we eat can all affect us. I notice this myself for example when I switched from almond milk to soya milk for a month (simply because it was on offer)- something which to anyone else is really no big deal but to my body my body caused very heavy periods and a lot of pain due to the increase in oestrogen. I only drew the dots when I switched back again. It’s not something that affects everyone in a similar way and just for you it could be something which seems very small a change or difference can have a big effect.

If you do see you’re GP, he/she may ask things like how long things have been different and to keep a diary for a few months- be prepared for this and if you can start a diary from now or backdate it as much as you can, this can help. If they feel that you would benefit from more investigative help such as a scan or it’s above what they are able to advise on, they may refer you to the gyne services they will also most likely suggest a diary for a few months so if you can start something like this it can help speed things up.
Log things like where you are in your cycle (day 1 is the first day of menstruation) what symptoms you have (such as pain, sensations, energy levels..etc) and how they affect you on a day to day basis (eg are they bad enough to warrant help). This is what I was asked to do when I was referred anyway, it isn’t essential to have this but it helps them a lot to understand what the problems and gives a clearer picture.