Is this a baker's cyst on my knee, or something else?

A few days ago (maybe around 5 days) I started running again (it's been a while) on the treadmill and doing general exercises (squats, crunches, pushups, etc.). Yesterday I noticed tenderness behind my left knee (the medial side, although my hamstrings are sore as well) along with slight swelling. My entire body was sore and somewhat stiff all over, presumably from my body getting acclimated to exercising. Today my knee feels the same way so I have been trying to keep it elevated and have been using ice. There isn't necessarily pain on the backside of my knee, more of just a strange feeling; just like something is different, although it is tender when I touch it in specific areas. I have been reading about baker's cysts and am worried that is what I have though there isn't a visible cyst. Is it possible a baker's cyst could have popped up that quickly? Or could it be something else, or just maybe sore from working out? Any words of advice or other suggestions/information would be helpful!