During engine startup today on a C152, I did my normal procedures. The aircraft was previously flown a couple of hours ago, so I used about 2 shots of primer, I then proceeded to crack the throttle, master on mixture rich and turn the key. The airplane has a 10 second limit on the starter, so i turned the key and let it turn over for a few seconds and almost had it started. After this my instructor tried to start it and then added about 3 more shots of primer, and once it started it ran rough, almost as though it wasn't firing on all cylinders, it did that for maybe a minute and then it ran normal. My question is, what would have caused it to run rough for a minute or so, it has never done that before and just wondered if the extra shots of primer made it do this.