Why does the temperature of let’s say 85 degrees Fahrenheit in southern California feel hotter at 7,500 ft elevation than it does at sea level?
How things feel is partly psychological and often doesn't have a purely physical explanation. If you run a hot bath and the water from the tap starts to feel tepid, plunge your hand into the very hot water with which the bath is filled. Hold it there for a minute or two, then put it under the tepid tap. The water will feel stone cold, though when tested with the other hand it still feels tepid.
If 85F at 7,500 ft feels hotter to you than 85F at sea level, it could be due to a number of things. On a sunny day you will get more solar radiation at high altitude, especially around mid day, and that will be in addition to the shade temperature which weather forecasters use. When walking from A to B in the mountains, we usually exert ourselves more than we would at sea level, and therefore we feel hotter.
The day you have in mind might have been especially humid in your mountain location, so that would make it feel warmer as well. Maybe your walk was when the sun was at its highest and you are comparing it to walks in the lowlands where, although the shade temperature was the same, the sun was lower in the sky or maybe there was even some cloud. And always there is the psychological aspect to consider. Our minds like to play tricks on us.