Firstly, yes your low BMI might be making your condition worse. I suffered with runners knee while very underweight as well and my GP basically said my legs didn't have enough muscle to deal with the exercise I was doing. It wouldn't let up until I gained some weight but I've been fine ever since I got to a healthier weight. It's possible this doesn't apply to you if you're naturally thin and don't have any problems with food as I had quite pronounced muscle wastage but my guess from what your friend said is that your weight probably isn't healthy for you.
Secondly, to fix most running injuries you need time off for the problem to heal. You also need to try and fix the underlying issue - in my case this was weak quads, might be the same for you or might not. You'll do better to pick your training up in a few weeks when you're pain free than to keep going and make the injury worse - that will only leave you in pain during your race or having t take time off closer to the event.
finally, you say you're running 5 miles a day - it's much better to do different runs - some long, short, fast etc and take some rest days than to do the same thing continuously