Feeling uneasy after confession

I went to confession earlier today and there was a mortal sin that I wasn't even sure I did (basically I wasn't sure I even did the action and I thought I was remembering things wrong) so I chose not to confess it. If I was sure I did it then I would have confessed it because I have confessed things much worse in the past. While I was saying my act of contrition I began to feel like I actually did the the mortal sin and that feeling has persisted ever since my confession. I would like to receive communion tomorrow but I am not sure if my confession was valid. Please give me some advice on what to do next.