1999 chevy Malibu, rear end fishtailing, then major wobble

The last 2 days my 1999 chevy Malibu has
started to fishtail as im driving down a dry
straight road with very few bumps, it starts
to pull to the right and when i slightly
correct it it'll veer left alot more than i had
anticipated so i left off the gas and it feels
like i just hit ice and my rear end starts
sliding all over the place and i have a hard
time regaining control, i left my boyfriend
take over driving today and it started doing
a major wobble like one of the tires were
about to blow but stopped when we slowed
way down, all lug nuts are tight as possible
and all tire pressures are exactly where their
supposed to be. Can anyone tell me what is
happening with my car????