There could be loads of reasons for this - not eating enough is just one of them. You might be coming down with something, be a bit stressed etc.
I exercise 6 times a week (by exercise I mean an hour plus workout rather than taking the dog for a walk). There are days where I get back from the gym and feel like I can barely walk!
You might have just overdone things a bit. You say you exercised before, but you've upped your routine and, I assume, reduced your calories. It could just be your body getting used to your new activity levels.
I wouldn't worry about it too much. If it keeps happening then maybe mention it to your doctor. Otherwise, give yourself a day off and make sure you alternate your exercises so you're not hitting the same muscles day after day.
Your tiredness could also be caused from your sugar levels being too high or unstable?
Finally, well done on the weight loss! 11lbs in six weeks is great
Em