Have you been doing any new exercise lately? Longer walks? Jogging? Heck, even different shoes? You can get strained feet by prolonged walking in "toe-thong" shoes like flip-flops, since your toes actually have to grip with each stride to keep the shoe in place, even if you are not aware you're doing it. Essentially what you're creating is repetitive strain that can lead to injury. Also, high heels can be a major culprit. If you've been to a wedding lately and danced in those high stilettos, not surprisingly you might have painful feet.
But even if you haven't been doing any of the above, I wouldn't jump immediately to joint pain/tendonitis. The initial swelling to me suggests an injury of some sort, even if you aren't aware exactly when you did it. Sometimes these things just happen. For instance, my mom walked around on a broken foot for days. It was chalked up to a stress fracture but to this day she doesn't know when it actually happened.
A few years ago, I strained the tendon on the top of my right foot, probably a combination of starting a new jogging regime and not having the best supportive running shoes. I also don't know exactly when it happened, just that I started noticing my foot was quite painful, especially when I initially got up from a seated position. This lasted for several weeks. It eventually subsided and hasn't been a problem since. It didn't bother to mention it to my doctor, but in general, soft tissue injuries such as muscle strain can take as long as 3-12 weeks to heal, so keep that in mind.
Basically, sometimes muscle strain is just muscle strain.
It's easy to want to blame everything on our UC, but we are just as prone to the normal everyday problems other people face. That being said, if the pain persists, worsens or if you are really concerned about it, it wouldn't hurt just to have your GP take a look.