My knee hurts. What's wrong with me?

So my left knee gets achey every now and then. It doesn’t seem to have anything to do with the weather or anything else I can think of.

Yesterday it was hurting a little in the morning, and I went out for a little while, and when I came back, it was soooo sore! It hurt to bend it, it hurt to keep it straight… And it swelled up as well. Not a ton, but enough that I could feel the swelling and notice a difference between my right and left.

I took some ibuprofen before bed and that seemed to help a little, but today it still hurts. I didn’t slip or trip or do anything to injure it, so what do you think? Arthritis runs in my family, so do you think that’s what it could be?

Thanks in advance for any insight.