I know; it's hard as newcomer to the hobby; so many things to learn!
In what we call a 'cycled' tank, there will be good bacteria living in your filter that eat the fish's wastes and stop the water becoming poisonous.
When you changed your filter, you will have lost all those good bacteria, so the toxins (ammonia and nitrite) from the fish's poo will be building up in the water and making them sick.
You need to do a big water change, making sure your new water is warm and dechlorinated. Try and change at least 95% of the water right now; leave just enough water for the fish to swim upright (don't forget to switch your heater/filter off first!) before refiling.
If you can get some test kits, for ammonia and nitrite, that would be great, so you'll know how bad the water is and how much you need to change. If you get your fish shop to test the water for you, get them to write down the actual numbers for you.
Until you can get your water tested, you should change at least half the water every day.
I feel I ought to tell you that some of these fish might die, as they might have been poisoned too badly to recover, but lots of fresh clean water is the best thing you can do
How big is your tank?