I had sex on my third day of my period... can I get pregnant?

My period is really irregular it’s 5 to 7 days cycle. Sometimes I get it within 2 to 3 months. But my last, it was like last month. Right now I’m menstruating. I get a normal flow. Today, fours hours after having sex I had a discharge going to the bathroom. I’m thinking about getting Plan B, just in case.