Firstly you're not being stupid to ask.
Secondly I think it has nothing to do with your cutting whatsoever.
It could be that you've pulled a muscle but then you'd get a bit of pain in your thigh probably down the back of your thigh/ at the back of your knee as this is where your hamstrings tun and those are the muscle group that contracts to bend your knee and stretches to allow the others (your quadriceps) to straighten it. An injury to a muscle tends to make them 'shorten' so you'd get cramping sort of pains if your stretch them by trying to walk normally. You could also have slightly injured one ofthe several ligaments in your leg especially if you slipped or something and twisted your knee but these would tend to just cause pain when you had weight on your leg and not particularly affect straightening. Its probably worth resting it, taking ibuprofen if its swollen and this will help with any pain, if it's painful after that you could try some paracetamol too. If it becomes swollen then use RICE (Rest, Ice, Compression, Elevation) and if it doesn't start to improve within a couple of days see a doctor.