Hey, it was still a thermal. You either missed the core by some amount or it was a small "pre lifter" that came along before the big one. I've seen that from time to time as well.
But you also now have a little more knowledge of what it feels like in the battle field instead of just the theory.
Things you can also do when it's warmer is to watch for insects coming up out of the grass. The warm air encourages them to fly and then they are lifted up. Also small groups of swallows or other small birds darting around in a "furball" is a sign of a thermal as they are eating the bugs that got lifted out of the grass.
Finally there's the "cheater's way"....
When they are in season find a few ripe cat tail flowers that are just shy of exploding into clouds of seed fluff. Wrap the "hot dogs" tightly in newspaper and crimp the ends for storage. To use them at the field simply peel back a bit of the paper and tease out a few seeds with your free hand and blow the loose seeds into the air. Watch them as they move around. Wait for the lull and give it a few seconds. If you blow seeds up into the air just a little before the "fill" blows in you'll see them begin to mix around like they are in a washing machine. THAT is your core and aiming point for a low altitude thermal that is still attached to the ground. When you find that sort of effect you don't even need to throw high and hard. You can actually just loft your model up into this maelstrom so it recovers at around 20 to 25 feet and watch it circle away to Never Never Land.
Since cat tails aren't really in season at the moment another option is fine wood sanding dust. Get a small bag full from a sander and you can just throw small pinches of it into the air and watch the particles for the same signs.