Yes using a fan can make you hotter instead of cooling you down.
That is true especially if you're fan is blowing hotter air on you and not cool air.
if the air in the room is hot already then all you're really doing is blowing hot air onto your body which can increase your bodies temperature.
But a fan works by creating a wind chill effect to cool your body down.
The fan helps evaporate sweat from your skin which cools your body down.
The more sweat that is evaporated from your body the more you cool down.
So fans while they can make you hotter they do cool you down also some so using a fan to cool down is better than having no fan and sweating it out.
If you were to sit out in some shade such as under a shade tree on a hot day and also use a fan then you'd be much cooler.