False negatives?

I'm currently 15dpIUI and I NEVER get symptoms for AF. (I don't know when she will come until I have ruined the underwear I'm wearing haha.) but lately I just haven't felt myself. My boobs are sore and my nipples are always hard and are darker and growing hairs around them. I'm bloated like no other and have these weird cramps and it's always on the same place on the left side. I'm nauseous and puky and heartburn really bad. I'm exhausted. I just FEEL pregnant. I have had one faint + on a first response but the rest after that have been negatives. Could it just be too early? Could I be pregnant and not just showing on a test? I had a previous pregnancy that didn't show up on a test until 5 and a half weeks. I don't know what to think? I'm going to try to get in for a blood test but I'm just so confused about my body.