Back of left heel pain

First off, I'm a totally new runner. I just started running two months ago, first week of March, and recently completed the C25K program. I'm male, 32, 115 lbs, 5'1" (yes, I'm Asian and fairly petite), and am training for my first 5K run on May 18th, about 2 1/2 weeks from now. I run 3X a week and do cross-training once a week, twice if I'm not lazy. I've been really careful not to get injured, but last Friday, after my 3.1 mile run on the treadmill, I went hard on the elliptical. My target heart rate is normally 148 on it, but I increased it to 160. After my usual 30 minutes work out on it, my legs felt a little sore. The next morning, the back of my left heel started nagging me. After this, I'm a little scared of the elliptical. I haven't used it since that night. It doesn't hurt much, but when I jog/run, it's throbs slightly, but not enough that I need to stop training. I thought the pain would go away, but just tonight, I couldn't even finish my 3.1 mile jog. I stopped at 2.5 miles and had to walk for 2 minutes, then finally jogged the rest.

What should I do? Advice would be greatly appreciated. I just don't want to have to skip my first 5K due to the nagging pain.