Serious gum & tooth pain 5 days after whitening treatment?

I used a Crest White Strip 5 days ago. I have been in the worst pain everyday since. I don't have sensitive teeth. I have used these strips probably 100 times. I haven't used them in 4-5 months, but I left the strip on for 20 minutes, even though it calls for 30, since I hadn't done it in a while. I don't understand why my gums are so terribly swollen and sore. Every time I brush my teeth, they bleed and today (the 5th day) even my teeth hurt. I don't know what to do at this point.