Hi, I Have Been On Birth Control Pill Form Since

Hi,
I have been on birth control pill form since July of 2012 after having my last son, as badly as I hate to admit it I have not been very consistent with taking the pills. Here for about the last few weeks I have been feeling fluttering like feeling in my lower abdomen, not the feeling of gas or anything else. I have taken two home pregnancy test both with a negative result. I know usually it means I'm not pregnant but I just feel completely different. I have had negative urine and blood test performed by my doctor before with my second pregnant up until I was 4 months along due to low HCG. Please help me and give me advice.