Is it just me or does this happen to everyone?

In all honesty i haven't been training properlly for a good 6 month now. I recently managed to start going to my kungfu classes again. Except i seem to be really out of shape despite having been to the gym fairly frequently. I'm not sure if it's because my cardio is just crap or maybe i have some medical condition?
I don't run much because i get shin splints. This has got me on a cross trainer instead. A typical cardio workout would be 2000m warm up on the rower trying to beat the time i done it in the last session. Then on to the cross trainer for 15-20 mins set on the hill program (the resistance level starts low gradually getting higher peaking at half way then back down again), with at least 10 30 sec sprints throughout. At kungfu classes we usually run through a 15min warmup & stretching routine, then on to basic kicking and punching drills by which time i'm starting to feel sick and lightheaded.
I don't know whats going on and it's affecting my return to training. It feels like i've taken a step forward but then 50 steps back. The only other time i've really experienced anything similar was when i was a kid doing p.e. at school.

If anyone could give me some advice on this it would be appreciated. Maybe offer some ways to improve my cardio? Or maybe i should seek medical advice and get checked out?