It's still early..

I started feeling nauseous about 3 weeks ago. Day one I thought nothing of it since people around the workplace had been getting ill and I just figured "alright, it's my turn to get it". Had the same feeling for 2 more days. Then it goes away for the weekend. No fever, no chills, no illness. Since then I've been feeling nauseous quite a bit. I have 2 kids, so I know what pregnancy symptoms feel like. The problem is that I have been using condoms every single time my dude and I have sex, so it can't be pregnancy, can it? I am a single mother of 2 and doing anything without a condom is out of the question. I'm not late for my period either. It is due later this week but I'm not late. I've been getting headaches on and off but as far as other symptoms I can't really tell. Being a single mother (kids 5 & 2) as stated before, I'm usually tired and irritated anyway. Breast are not tender or anything like that. I just don't know what else would cause what I'm going through. I took a test 2 weeks ago and it was neg., I'd figured that anyway since I was far from being late. I'm not dehydrated because I can cry tears, I know cause I was crying earlier when I puked up my lunch after I got home from work. But, I will wait for my period and then if it doesn't come by Mon. I'll try another test. I know my senses tell me I'm pregnant, but how is it possible with a condom EVERY time not only that but how is it possible that in the case that I am I started feeling it (nausea) about 1 week and a half after my last period? Please someone talk to me.