Sex nausea?

Hey, I have an ongoing issue. I'm an 18 year old girl and my boyfriend is 20. We have been together for 2 years and 4 months. We started having sex a little bit after our 1 year anniversary, but I have had the implanon since a month into our relationship. Because of the birth control, I have always let him cum inside of me, but it always makes me feel extremely nauseous. I get incredibly sick afterwards and have no idea why. I've googled it and come up with nothing aside from a possible allergy, but I don't know if that's right. I have thought about talking to my mom about it because her and my father both know we are sexually active, but don't know that we don't use condoms. Any ideas on what could be going on with it?