I'd give him the benefit of the doubt this time and then if it turns out to be an on-going problem address it later.
After a very long period of not doing anything physically demanding - I started working w/ a trainer around this time last year. She has a policy that you have to commit to 3xweek for the first 3 weeks and then you take it from there as to how/whether to continue. In the second week we did a really hard workout - my arms were so fried I needed both hands to pick up my water bottle.
Then I was sick over the weekend - flu or something, but I didn't want to cancel the monday workout, so I mentioned I'd been sick but was feeling fine and wanted to continue. We did a not terribly taxing workout on that Monday. On Tuesday my arms were quite swollen, but, hey, I thought "I'm Pumped!" [feel free to laugh]. Wednesday the swelling was worse and was starting to spread to my hands. I went to the infirmary at the University where I worked. The doctor said it was probably just really bad weekend warrior syndrome, but he couldn't rule out the possibility of deep vein thrombosis.
So, I spent most of the evening at the local emergency room. Not at all a fun experience, esp. since they couldn't get a good stick in either of my arms or my foot (I'm a long time blood donor and it's never been a problem), so I ended up having a groin stick so they could get a blood sample. The one funny part was when the technician was ultrasounding my arm said "wow, there's a lot of swelling here!" I felt like, "yeah, that's why I'm here."
Anyway, it turned out to be just overuse, my doctor lectured me on the importance of building up more gradually and I'm now learning the olympic lifts w/ the same trainer. But, the point of the story is you don't want to get a call "hey, I'm on my way to the hospital, so I'm afraid I won't be able to make my session tonight."