could be due to low hcg levels
same thing happened to me my soon to be doctor friend had felt for hearbeat so i am now 10 weeks lol
been midwife, she thinks i could be further along, and i never even told her about my concerns..she just came to the conclusion by herself lol!
got my first EVER scan on 18th April really excited:)
4 weeks is a really big difference to be off. Have you been to the dr? The light period you are describing could have been implantation, which means a BFP may not have shown up right away. An ultrasound can usually measure an estimate of how far along you are.
If you had negative pregnancy tests and had a period even if it was off I doubt you were pregnant then. Most likely you are only 6 weeks. Best thing to do though would be to talk to your doctor and get an ultrasound.