Possibles are as Menhir said, if you did not have a gauge, then you have not been checking the pressure.. So most likely the pressure has gone down over time, bumps here bumps there, getting heated up then cooling down.. and time all these will lower the pressure in you tires.. So check the front tire as well, my guess is that it is low as well.
So after, how long have you had the scoot?? you hit a bump or something and it broke the seal around the tires edge enough to let the rest of the air pressure out. Maybe you came off the curb, the rear smacks the ground pretty hard when you do that.
As you have learned the hard way "Tire pressure is very important to maintain" It is something you have to do regularly. You do not have to check it every day, but atleast a few times a week should do. And remember when your checking it every hiss of air you hear is some pressure getting out. Also a little gets out as you put the pump on, and take it off. So it would probably be a good idea that atleast once every two weeks or if the tires get more than a few pounds low that you should pump the air pressure up over what it is supposed to be, Take the pump off, then check it with the gauge and bring it down to the correct setting. It is easier than it sounds, and it does not take much more time.
Since you let this go, I wonder if you have checked the oil, how many miles are on your scoot, is it time for a service.. such things like that. It sounds like your fairly new to scootering, so do not worry too much about not doing this stuff, but you do need to learn how to check these things, as it will help you to have a much safer scootering life. If nothing else hook up with Jimc and see if he can teach you how to maintain your scoot, so you can take care of these little things that always crop up.
If the tire is holding air, I would try to ride it around the area for a bit and check the air again, if it is holding then ride around a bit more. Once you get back and it has still held air. Then check again tonight, and again in the morning. Remember if the pressure is at say 26lbs tonight that it may be a little lower in the morning due to cooling down overnight.
Good luck to you!!