Period at the doctors..

Please read all the way and help me out!!

So I went in today 2/15/17 to the doctors office to see if I had a bladder infection. Well they took the urine sample like always and came back and well when they asked when my last period was I told them it had just started yesterday. The doctor came and told me what he was prescribing me and went on talking about what it was and how long to take it. The nurse who had me take the urine sample came back in after my doctor had left to hand my the papers about the prescription and what happened in my appointment and she had a confused look on her face and said "If you just follow me I'll take you to get some blood work done for us" didn't mention what it was for. Well me and my boyfriend went over with her and she had another nurse take care of me and sign off on the consent to treat and privacy laws( I think that's what she called it) and before I signed it I asked what the blood work was for and they said it was a blood pregnancy test. Which was weird because they've never had me do one before and with the confused look on the first nurses face I don't know if it was maybe a little blood ran into the urine sample and it was messing up the results or what. And to mention my period started off very heavy the first day and it's never heavy the first day. It's usually light the first 2 days.

Can anyone please tell me why they might be doing a blood pregnancy test with this kind of a situation? Why they might look confused or maybe concerned? Someone help me!