nausea & no explanation ??

so i'm on birth control & nausea is part of the symtoms but it's never been a major factor for me before. my parents think it's from the heat because it's been 100°- 110° outside. i've been drinking a ton of water so i can't be dehydrated anymore. i'm sexually active w. my boyfriend, but i don't think i'm pregnant because we've always been safe about it. i'm really confused as to what could be wrong. any suggestions??