One pec is really swollen

I tried running a search but most of the threads just involved people who had one pec lagging behind the other. That's not the case with me though. I just started working out last week...and since doing chest, one pec has become noticeably larger than the other...looks like its swollen whereas the other one is normal. I can definitely feel the difference in size as well. It can't be that I'm working one side of my chest harder because I just started last week (and I haven't touched a gym for about 10 months before this).

However, I do remember the same thing happening to me last year, where after getting into a work out routine, my left pec would really look swollen and become larger than the other, though I think it subsided in about a month.

Any idea what this could be? My gym buddy (who by no means is knowledgeable) seems to think its an injury....I somehow damaged my left pec. Although it doesn't really hurt (aside from the usual post workout soreness) any more than any other muscle. I dont know if this extra info helps but I'm 23 and on the slim side (150 lbs @ 5'9)