I never feel full anymore, help?

So I started eating less, which I know I shouldn't have but I started to be hungry almost all the time. Now whenever I do eat a decent meal it just doesn't fill me up. I get full for maybe 5 minutes of the healthy fullness and then I get this weird empty but full sensation in my stomach that just isn't satisfying. It happened this morning after i had a cup of cereal two pieces of bacon and half a pancake. This is a normal amount of food and I got the weird sensation yet again. What does this mean? Why am I getting it?