I Guess My Body Wasn't Ready for HIIT Yet!

I've been doing about 20 to 30 mins of moderate exercise 5 or 6 days a week on my stationary bike for about 6 months now. A few wks ago I read about HIIT and got all excited about it, so I started doing a 10:30 min workout - 2 mins moderate, 30 secs HI, 2 mins mod, 30 secs HI, 2 mins mod, 30 secs...