Word for "sitting down heavily?"

Solution 1:

Mom came over and plopped on the bed, making my head bounce on the pillow.

Or dropped.

• To let the body drop heavily: Exhausted, I plopped into the armchair.

Source: The American Heritage® Dictionary

Solution 2:

Mum came over and plonked herself on the bed, making my head bounce on the pillow

• (often foll by down) to drop or be dropped, esp heavily or suddenly: he plonked the money on the table.

Source: www.thefreedictionary.com

Solution 3:

I would use flump because it sounds like a noise the action might cause, and evokes both slump and flop simultaneously. Oddly, it appears Flump is also the name of a British marshmallow candy, which might confuse candy aficionados.

Flump v 1: To move or fall suddenly or heavily

Solution 4:

The word "slump", that is, to fall swiftly, far and suddenly, comes to mind here. It may be a bit strong, depending on the exact context. To "slump" is very near in meaning to "collapse" and, when said of people or other living animals in a context like yours, is wont to bespeak someone very tired, exasperated (maybe even near to a nervous breakdown!) or undergoing a medical crisis like faint, heart attack, cerebral ischaemia, having swallowed a cyanide suicide pill or having been shot through the window by a disgruntled neighbour ...... (let your imagination run wild). So, if Mum wasn't very tired or being picked off by violent locals, look for something else. However, slump, IMO is a wonderful example of onomatopoeia.