Could I be pregnant ? Or Overreacting?

So this past week August 1 to August 6 I was in my fertile period. From August 3 to August 6 I had sex with my boyfriend [ even sometimes more then once ]  Everytime right a second before he ejactulate he pulled out. But now I'm having some slight not really noticeable pains in my lower stomach, along with nausea. Even drinking water sometimes makes me nauseous. Those 3 days I had 13 to 33% of getting pregnant. Can I be pregnant? Even if he didn't ejactulate? And my period isn't due until August 20.