What could it be? Tender breasts for a week after periods but I am on copper IUD.

[cherryblossoming]

I had periods about two weeks ago. A few days later I started noticing my breasts had gotten slightly bigger (just slightly) and they are tender.
I did Tesco pregnancy test this morning and it is negative. It worries me a lot. Usually women get sore breasts before periods but not after. My copper coil ( do not know the exact name and brand only that it is copper) was fitted in just over 4 years ago. Last year I stopped feeling threads. They did the scan it showed it was still inside. The doctor has assured me that it is working.
It gives me lots of worries. Shall I wait for my next periods or go to the GPs before that? I am aware of the ectopic pregnancy.
Thank you for any suggestions.

Anyone?

I had an ectopic pregnancy with a copper coil fitted and I did show up positive on a pregnancy test but only after about 7-10 days so I would maybe test again in a few days. The other sign for me was I was spotting dark brown blood so look out for that. I get sore boobs around ovulation so it could be that though? Go to the dr if you're worried though as they can do a blood test that will be able to confirm anything.

Thank you Sophie. I think I will wait one more week and then by a pregnancy test or go a GP. I know they do blood tests but they are not always willing to do it according to some ladies here on mumsnet. I do not know what I am going to do if I am pregnant. I have been thinking of looking for a job.