Breast lump?

Last night while lying in bed I found a lump under my breast.  It's hard and painless but about the size of my thumb.  It's under my right breast and just about where my breast meets my chest.  After I found it I decided to have a look in the mirror to see if I could see anything but I found that when I stood up it seemed to disappear.  I can only find it while lying on my back.  

I am trying to decide about whether to go to my GP or not.  I don't really like him and would rather not go if this is something minor.  Is it normal for breast lumps only to be able to be felt while lying down?  Is it possible it's something minor?  Does it matter that I can't find it while I'm standing up?

I'm 37 and female.  I'm generally in good health although I do have PCOS.

Any advice would be much appreciated.

Definitely schedule an appointment with your GP and have it looked at. Is it possible that it's something minor? Yes. But any hard lump should be examined. If it is something more serious, you want it removed as soon as possible. But don't freak out. Hopefully it's benign.

Its recommended that you lay down on your back to examine your breasts so as you have found one,you should be making that appointment with your GP.  If you really dont like your GP could a practice nurse examine you?  most lumps turn out to be benign so dont worry, but please make that appointment.