I'm losing hair and I don't know what's the cause and how to stop it from falling off?

Every day I'm losing a lot of hair. I've been straightening my hair for about 1 year but I was using 2 different straighteners. I was using a $30 conair straightener for about 9 months, & then i bought a cortex one which was $50. When i was using the conair one, i wasn't using any heat protectant. But when i used my cortex, i used one which was cortex insulator, that was when i started losing hair. i noticed that the two straighteners are completely different, my cortex was pulling my hair harder than my conair. Both are ceramic & i didn't have any problems with my conair except that my hair is frizzy while using it. When I'm using my cortex, it's very healthy. i really need help. i always see hair over my sink & ever if I'm at school or any other place, i still lose hair when I'm not even touching it. Also, i can easily grab 10 strands of hair if i just run my finger through my hair while I'm in the shower, usually when I'm conditioning my hair not shampooing. i use pantene for both. Is using the conditioner bad for me? How can i stop it from falling off and will it grow back? Any answer will be helpful.