Installing New Shower Fixtures

Hi, this is my first post here so sorry if it's been asked too many times! When I tried flushing my toilet the other day, it ended up overflowing from the bowl. I thought it might be a clog from too much toilet paper, since I hadn't dropped anything in the bowl. Since then, I've been trying everything I've found online but haven't been able to figure anything out. I'm almost certain that it's not clogged now, but every time I try flushing, it seems as if the water that was in the bowl doesn't drain away and the refill just doesn't stop. Each time before I try, I turn the water supply back on and let the tank fill, but without fail every time I flush, it just starts adding to the water in the bowl at an alarming rate. I've tried moving the refill tube to be both in and out (aka directed towards the tank), as this has happened before and I think I remember trying that. No difference, however. At first, it would overflow but now that I'm keeping a closer eye on the water supply & flapper each time I try, it just fills to the brim with no water actually being flushed away. I'll then turn off the water supply and let it sit and within half a day, both the water in the bowl and the tank seem to have drained away completely. If someone could give me some advice, that'd be great - I'm really, really hoping that I won't have to completely just take the toilet out

This happened after I used [URL="https://www.homedepot.com/p/Instant-Power-67-6-oz-Hair-and-Grease-Drain-Opener-1970/100169339"]this[/URL]. It's a rust-colored stain. Tried CLR with no luck.