It is not likely that you are pregnant. Sporadic periods make it difficult to figure out when or if you are ovulating. Taking a pregnancy test three to four weeks after you've had sex should tell you if you are pregnant or not. So, since you had a negative test you are not pregnant. Pregnancy tests are very accurate these days with women getting positive results before even missing a period.
I'd not expect your belly to be hard/firm this early in pregnancy either. The uterus would not yet be up by your belly button, I don't think.