I've read that women that are pregnant can get sick pretty easy because their immune system is low

Me and my boyfriend had sex on January 19th just a day or two after my period ended. We did use a condom which didn't break. A few weeks later I started to feel tired for no reason. I was really streessed with work so I thought that was what was causing it. I got my peiod on February 9th and it was normal just like every other one ..lasted 7 days .. now I have a cold and I've read that women that are pregnant can get sick pretty easy because their immune system is low. Could I be pregnant or no? Help!!