Why are my legs burn so bad after 5 - 10 minutes of walking or running?

I am 21 years old. I have been running and walkin on the treadmil for about 2 years now. and usually burn betwwn 600 and 900 calories. between 40 minutes and 1 and a half hours. depending on if i was running and or walking. but for the last couple of months I have not been able to do more than 10 minutes burning absolutely minimal calories. Niw I have read in a few different places that it could be related to some sort of alergy. I have never before had any probablems with alergies until just a couple months ago i got really really sick. but after 2 weeks as soon as i was over it i attempted to get back into the gym. but why in Hell are my legs burning after 5 - 10 minutes of walking that makes no sense I dont even get a good sweat in anymore pleas if anyone knows whats going on and or how to fix the probablem and only if you do please let me know