Why do I keep getting nausea

I am 10 months postpartum with my second son. My husband and I have protected sex, but we aren't worried if I conceive again. So last Friday we had unprotected sex and he didn't go inside but he pulled out. Anyways I haven't had my first period yet I actually haven't had a period since August 2015! I am breastfeeding as well as feeding him solids. Anyways this last week I have been feeling so sick at night it's really strange. I really don't think I am pregnant but what could be causing this? It makes me cruelly feel like I could be sick. Has anyone had this? I have also being having a lot of heartburn Everytime I eat like WTF is wrong with me 😞