I Love Running

I'm pretty new to running, and I've been running into an issue (unintended lame pun). Lately, after a run, my left hamstring (behind knee) will always ache. I tried to run on two consecutive days after getting this, and it totally ruined my run on the second day, so I took a week off of running. I just ran again and felt great, but now it's back. I always stretch after running, but that doesn't seem to help other than with regular soreness.

Is this normal for inexperienced runners? (Will it go away as I build up mileage?) Should I just keep my run frequency low? Advice appreciated!