confused??

so I had my period the 7-12 my boyfriend and I had unprotected sex the 30 and 31. and some more earlier that month. I took a pregnancy test the 5 and then again the 7 and a couple in december and all came back negative. my period had clots and everything in it so it was a normal flow for me. lately these last few days i’ve been having lower back pain and cramping on my left side. is this normal? I started back on birth control this month so could that be causing my body to act all whacky? in december I didn’t take birth control because I was on antibiotics and I ended up having 2 periods that month. is there a possibility I could be pregnant if I had a full period with clots for 5 days? or is my mind playing tricks on me. i’m also undergoing the flu and possibly have strep again so I wonder if that’s playing into affect. should I buy another test or just wait it out?