Why when I straighten my hair it looks so frizzy?

Ok I have been straightening my hair for years and this sometimes happens to me. A few things come in mind. 1) your straightener sucks and is cheap, get one around 100 euros with top notch technology and behold, 2) your conditioner sucks and makes your hair more moist and so more frizzy, 3) the temp is too low, 4) you need a lighter heat protective spray. I recommend Alterna, it smells amazing and sprays fine like perfume, also rejuvenates.

Use kerastase shampoo and conditioner. My salon turned me
Onto it & it is hands down life changing. I swear by it. It’s very expensive but very worth it. You WILL NOT be disappointed. 

Maybe try some anti frizz products or a straightening balm / straightening cream to help keep it looking straight. Use products that are meant to keep your hair looking straight on top of using the heat protector. Heat protectors only protect your hair from heat styling tools, they usually don't do much to keep your hair from frizzing up. Also use a strong holding hair spray when you're done to help hold your style.
If / when you can get a hair cut get and get a good one. Get the stylist to remove some of the bulk of your hair, this can also help reduce the frizz.