Nausea feeling while having sex?

this has only happened three times. In the last week me and my boifriend have had sex and 3 times we've done it ive low kei felt like i was qona throw up. Also my tummy kinda feels a lil hard, a pair of shorts i put on a few weeks ago fit and now they are tight on me. ive also qotten a few head aches which i havnt experience in a lonq time. itook a preqnancy test but it was neqative but im iff on those test and think i miqht be pregnant but idont want to get my hopes up. could it all b in my mind or what