swollen knee

Yesterday morning during a trail run I stepped into a shallow area and sort of jammed my knee.
It didn’t hurt I just felt the knee didn’t flex and I absorbed the impact on a straight leg.
I didn’t think much of it until going to bed and noticing it was rather stiff.
I ran easy this morning and although it was a little stiff, I warmed up easily and had no pain. Later today though the knee became quite swollen, once again no pain.
I have iced with considerable relief. Any ideas?