Tubal pregnancy concerns


So I am having pretty severe stomach and lower abdomen pain. I went into my dr on Friday and he said stretching pain is normal. They have gotten much worse. My lower back is now aching & im having shooting pains. I went in at 6 weeks and had HCG levels taken... they were only 123 then 48 hours later 315 which is not very high. My doctor said I may not be as far along as I think but he only estimated me being a week later than I thought which would mean I still should have higher HCG numbers. I feel very light headed the past two days. When the pain comes it causes me to feel nauseous it's so bad. I'm a bit worried about a tubal pregnancy. Has anyone had one and could tell me some personal stories. According to first day of my last period I am 7 weeks 1 day. Any suggestions?