Why am I weak and very shaky after cardio and weights?

Ok guys and girls. I'm 24, an Army veteran, and a college freshman. Since I've ETS from the Army, I have gained approximately 40 pounds. From 165 to 205! Now that I am here on campus I want to get serious about my health so I began my old cardio and weight regiment that I used in the military. However now after I workout I feel so rediculously weak and trembly. It kinda scares me! I am not ually active and I eat 3 small meals a day of whatever I want. (small portions) Can anyone shed some light on this for me?