Knee painful for no apparent reason HELP!

[WhyDoesMyKneeHurt]

I'd love some suggestions please!
About three days ago I woke in the morning with a sore knee. No trauma to knee and it was absolutely fine the day before.
It eased during the day but now the pain is back. It's worse when I have to bend it.
What could the is be? What can I do?
I've taken ibuprofen...
Thank you for any advice.

[Summerslimtime]

Bursitis?

[WhyDoesMyKneeHurt]

Isn't there a lump with that?
Google tells me maybe tendonitis. Scary how you can just wake up one day with so much pain!