Am I overreacting or is this a legitimate reason to get worried ? I just want answers please !!

Angelica • just a little bunny takin it one hop at a time
I am 18 years old, got accepted to the university of my dreams and am not planning to have a baby at this point in my life...
Recently (a month & a week and a half ago or early February) I started using birth control. A week after starting I became sexually active for the first time. 
This week, Sunday and Tuesday, my significant other finished inside me and I am worried about the possible outcome that I will be pregnant. I haven't missed a pill at all but it has only been several times that I took it at random parts of the day and not the usual time....(it was once last week I took it not on time and the other times I can't remember when) 
Overall I need reassurance or an idea or an answer of if I am ok or not. Also I was planning to buy Plan B but it is expencive, so before I actually bought it I thought to write this. 
Please reply !!