Could be shoes, hip misalignment, anything. Most common reason would be trying to do too much, too soon and not enough rest days. Even 20 minutes is a long stretch after 0kms. I was in the same boat earlier this year and started by doing the C25K program. The first week you do more walking than running and you'll think "this is dumb, I'll never get to 5k or 30 minutes non-stop in 9 weeks", but it picks up while giving you enough recovery time and a surprising increase in speed and distance. My average week now is 50-60kms and last Saturday I ran a personal 10k TT with a couple of km sectors in the high 4minute zone for the first time (that really pleased me!). Bottom line: don't do too much too quickly. And check out the YouTube running channels - lots of useful advice there.
