I'd just keep doing what your doing for a few days to see how it goes. You may have twisted or pulled a muscle without realizing it. As long as it keeps getting less and less painful with no swelling, I wouldn't worry much about it.
Hopefully in a few days it will be a memory, not a good one, but a memory. Obviously if it gets worse or gets red or swollen, then I'd see a Dr.
Make sure you go cautiously up and down stairs till it heals.