That probably is a bonk, it's the same thing as over-exerting your self. You could have been over heated but I vote for bonk, too tired, whatever you want to call it. It sounds like a pretty normal bonk to me. It’s what happens to me if I bonk, but I have learned what to do now.
The trick is to learn to STOP riding when you start to feel that way.

Always have some calories available on the bike. For a bonk liquid calories get into you faster. When you start to get that tired STOP and get off the bike in the shade. Have something to eat and drink and be still. You may recover. I always recover with fruit punch available at the small convenience stores, it's fast and I like it. Gatorade, whatever, just get some calories in you and get out of the heat. With the right stuff you can recover and get going again if you are not too far gone. When you just begin to feel that way STOP. I get on the edge of bonk once in a while and I recover very fast with some liquid calories and stopping. The sooner you STOP the better. Solid food takes a lot longer to give you any energy.

STOP before you fall into traffic.