Hello 🙂 Several months ago I was bouncing on my knees on a trampoline and, whilst I didn't actually hurt my knee at the time, it sort of felt a bit odd, and later that evening began to ache. This just got worse with time and made me stop taking the dance class I'd just signed up for. I went to the doctors and they basically just said "It's probably inflamation in the joint, it'll go away in a couple of months" the first time I went, and the second time they had me take ibuprofen at the maximum dose for two weeks.
I'd say see a doctor of course, but suspect you may get the same treatment as me… 🙂
To be fair, ibuprofen was sorting it, as it takes the inflammation down for long enough for the tissue to start to repair itself (it's so hard to not use knees!), but I used to be asthmatic, and after one week, the NSAIDs caused a return of that, so I wasn't allowed to keep going with that. They gave me a physio referral (not that I've heard anything since, after 3 months), and suggested a few exercises to do to strengthen the muscles that protect the joint. They were:
Sit with legs out straight and feet flexed (not sure if that's important, or just how they drew it!). Alternate between pressing a leg downwards into the floor for a few seconds, then lifting it a couple of inches off the ground (still straight).
Since the other exercise was basically to stretch your legs, I think stretching should probably be ok in moderation!
Anyway, my advice would be, see a doctor even though they'll probably tell you it'll sort itself out in 3 to 8 months (aaaaaaargh), and try ibuprofen at the max dose (3 times per day) for a fortnight if you don't have asthma or other contraindications. Remember to eat something when you take the tablets, even if it's just a biscuit and a glass of milk.