It could be that you are allergic to metronidazole (and maybe relatives?) because of your rash, but I'm not sure about it. So you have to take this in account with if you take antibiotics of the same group. But I don't know it exactly.
Taking only flagyl is not good, because it is probably not strong enough and they can become resistive to some sorts of antibiotics.
You should not smear gell into your butt, the parasites are also higher into your intestine. You need to take some oral antibiotics. And take the right type and ammount! You need to destroy the cysts and other stages of the blastocystis, so you need to take a combination of antibiotics to do this. Take out the amoeba form and the cysts will hatch later on and you are reinfected with maybe a resistant form of blastocystis.