Why do I begin to feel weak and achy during a workout?

I have been exercising regularly for the past month or so but in the last 2 weeks, I've found that I can't do it as much anymore. I used to at least 5 times a week, but now my body starts to practically break down in the middle of my work outs. I start to feel extremely weak and like I can't go on any further. Some days, I'll get nauseous as well. I start off with about 35 mins of cardio, and then a 20-min arm workout, which is usually the easiest part of my workout but now I sometimes can't even lift the weights anymore and sink to the floor because it hurts. Sometimes I'll even start crying.. It's pathetic! I don't understand why I'm feeling like this. Any answers?