Im really scared. my period lasted two days. what can i do? we used a condom and all the tests came out negative. can the change be due to stress?

Absolutely!: It sounds like you got a 2 day period after you had sex using a condom. The fact that your period only lasted 2 days is not uncommon particularly at your age. I doubt you are pregnant. However, the stress of unwanted pregnancy sounds to be significant & can affect periods. Please recognize you feel this stress even though you are using contraception. I would consider more reliable methods:pill,etc