Problems when exercising (symptoms inside)?

Ok, so every so often when I am exercising (not every time) I will get extremely light headed and it will feel like my muscles are LOADED with lactic acid, even if the exercise has only been a few minutes when these episodes occur. It will then feel like I am about to pass out and like my legs are failing, and my heart feels like it's beating kind of weirdly.... it's pretty horrible... Any answers as to why this is happening?