Negative pregnancy test but lots of symptoms?

Okay so I am 20 years old. My fiance and I have been together for a little over 2 years, and have had unprotected sex for the last year and a half. I say unprotected- I am on birth control, but I am very forgetful and always forget to take it on time. We are not scared of me getting pregnant, so if I am that would be wonderful! However, I generally have regular periods- I start the 3rd Thursday of every month. This month, I started 2 weeks early- it was very light spotting and lasted about a day. Since then I have taken 4 pregnancy tests and they all come up negative. I have many symptoms though, such as nausea, diarrea, tight stomach, fatigue, lower back pain, and breast pain. I am supposed to start my period tomorrow and have not shown any signs of my normal PMS, but since it was 2 weeks early I am not sure whether to expect it or not. I am a little overweight- not unhealthy, but enough that I think it might affect the way a pregnancy test reads the HCG levels in my body. Has anyone had a similar experience and do you have any suggestions? Like I said, we’re totally ready and it would be a blessing to have one- so either way it’s fine with us!