Desperation while swimming?

I just learned to swim, I can go a few meters free style, but sometimes I feel like I have to get back to the stand up position... I feel like I'm out of air, some kind of pain in my core, not real pain but something like a vacuum, I get desperate and I have to get out, and breath normally to recover. I don't like it cause I wanna be able to swim hundreds of meters. How do I get rid of it? Is it because I take too much air? Is it because I don't exhale the whole thing before taking the new breath?? Help is appreciated!