Most likely your holding tank vent is blocked. Tank vents have 2 main functions: 1. To provide an escape for air in the tank as it's displaced by incoming contents. If air can't escape, the tank--the whole system--becomes pressurized...and 2. To provide a source of air to replace contents as they're drawn out, without which the pump
will pull a vacuum that won't let it pull out more than a gallon or two. So if a pumpout or macerator pump can't pull in the air it needs through the vent, it'll try to find another source...which in your case seems to be the toilet bowl.
So do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning
out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck
pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...replace the vent line.
If there's a filter in the vent line, remove it...once a filter gets wet--which can happen when a tank is overfilled or even due to heeling--the charcoal swells and blocks the vent...one of the reasons why vent filters aren't recommended for use on most sailboats.
If you need more help to do this, I'll be glad to provide it...here or "one on one" via email