Seriously don't stress.
You definitely haven't lost it that fast!

Bad runs happen for a myriad of reasons, everything from a bad nights sleep, to shoes being to tight, to wrong food eaten, different time of day to what you're used to, dehydration, over hydration, wrong clothes, too hot, too cold, too windy, not windy enough, gone out too fast, different route had a hill you weren't expecting, hell I think some days it comes down to whether or not you're holding your tongue on the left or right side of your mouth.
Personally I could go out and run a ok HM nearly any day of the week (assuming I haven't hammered myself a day or two before) yet I still have runs where 1km is a struggle, not even 1 mile. Some runs I've pushed past this by using the old "I'll run to that lightpost and then I'll see how I feel." I read somewhere you shouldn't give up at the first thought, but at the 3rd, so that's generally what I'll do. Push myself to a lightpost/sign/etc, re-*kitten*, do that 3 times over in a row and if my head is still screaming to stop, then I'll stop. (Injuries and niggles a side)

You've got speed in your head, so it's entirely possible you went out too fast. Nothing wrong with going back to some of the C25K sessions if you think they'll help, though personally I'd move onto the B210K sessions if you need that interval breakup and a program telling you to do it. Some people need the support of a program to keep them going.

If you want a faster 5k, you really do need to get your endurance up to an easy 10k. Chasing speed too soon is the quick road to injuries.

3 miles will become easier with time, if you run it at an easy pace.
Once you honestly believe you can do what you're trying to do (Whether it's 5k, 30k, or 1k intervals) and you learn how to pace yourself for what you're doing you'll be laughing.

I'd second the view that you're probably going out too hard. If you're using something like Endomondo or Runkeeper they can give you pace every 500 metres so that helps keep on top of it.