Could be anything from that description. Doesn't sound like a knee problem tho, probably muscular.
Quads and hams could pull on the front and back respectively. Probably won't be both though.
Stretch the quads and hams (unless too painfully) and see what feels tight, that should identify any muscle issue and may help a bit. If any stretch finds tight muscle you have found the problem. If so keep doing it a few times a day and see if it eases.
If it persists get it looked at. Don't waste time with the doctor, see a decent physio or a sports injury masseur. If you live in southampton I can recommend either, pm if interested.