I've been having trouble lately with enemies fast expanding, but having such a massive ball of units that I can't contain them. It usually occurs during my transition to Hydralisks, so I'm still a little skimpy on units.
Tips?

Fast expand sacrifices units for the expansion. The only real "Fast Expand" is the Speedling Fast Expand, and that can be contained with roaches, or your own speedlings.

Fast expansion just means good opportunity to win faster. If you see them fast expanding, don't expand. Build more units. Go straight in.

So, are you going 1-base-hydra or why do you think this is a fast expansion? If you are, don't, get an expansion. If you aren't, well.. You should have even bigger ball of units.

Seriously, most often I have trouble containing opponents is when..

A: I screw something up. Be it making a bad counter or not microing/macroing well. Or not scouting the correct places.
B: The enemy has Siege Tanks with Siege Mode upgraded.
C: The map has a lot of narrow pathways they can block with bunkers etc.
D: I am about to lose anyway, because I have fallen so much behind in unit production.
E: The map has an expansion to which there is no ground access other than through their main.

Alright. For Fast Expansion.
1. Your opponent will not have a tank if he expands fast. If he does, then he will undoubtedly put up a bunker if he's Terran- leaving him dangerously troopless at start. You make sure that he gets it. If you're Zerg, try going fast mutalisks. They'll get hit hard and if you do economic damage, try not to lose your mutalisks. They're fucking expensive and every one of them lost is a gain for them. Play safe, and play to the standards.

2. Fast expansion Toss. He's probably going to cannon up. Roach in, get your speed upgrade as fast as you can, and then boom. Go in with lots of zerglings. GG

3. Fast expansion Zerg. Mass Zerglings. Hit before Muta timing.

I'm sorry, after I posted this I watched replays of some pro players (DIAMAGA or however his name goes). And I've been mending my problem. I'd been making very fundamental problems, like teching too fast without the troops to cover it.

You need to post up replays of you playing in order for people to help you out

I've been having the same problem, that's so funny, but I couldn't figure out what I was doing wrong until I actually read this. Thanks a ton man!