Knee tickle/soreness – please advise

Well, I’ve never had any kind of trouble with my knee before, never been injured period, and I’m not sure what to make of it, so I would appreciate anyone who knows more about knees than I do (which is nothing). Sometimes at the end of a long run for the last mile my knee would tickle this spring. I just thought it was kind of like the way a blister tickled, just ignored it. So one day it started “tickling” one mile into a 13 mile run. Like I said, I’m stupid and I didn’t equate a strange tickle with something being wrong, so I finished the run and the next day it hurt to walk. Basically when I bent my right leg after pushing off of it was when it hurt. So I tried to walk with a straight leg. I rested it and after some time off did an hour run where it became sore, so I headed back. Within a few hours it hurt to walk again. I rested it even longer this time and then decided to try an easy 6 miles, but this time it went straight from being sore to hurting while I ran. So it is a couple days later and still a bit tender when I press on the kneecap or along the outside of the knee. It still kind of “tickles” around the edges of the kneecap. It feels absolutely fine when I walk, but when I have my leg bent, like sitting here at the computer, it starts to become irritated. I have iced it and rested it but it continues to be slightly sore, especially climbing up ladders and such. It worries me that each time I tried running again after resting it a decent amount it became hurt quicker after I ran, starting will a period of 12 hours but then down to zero. I would appreciate if anyone can advise me what this could be and how long it takes to rest it before I can run again.