My first pregnancy was fraternal twins, im now 6 weeks pregnant. can i have lost a twin and that cause my levels to drop and rise again?

Yes: An early twin failure is not uncommon especially since you are at higher risk for fraternal twins.

No way to know that: That seems very unlikely but there is not a way anyone could determine such a thing by analyzing a bhcg hormone level. I am not sure how thinking about this remote theoretically possibility would be helpful either. I would focus on the health of your current pregnancy.