Mazda Spaceship?

Hello all! I have 2007 Mazda 3 Hatchback 2.3 L.
Yesterday I was driving home from out of state to
my home which is about 170 miles. Just about 110
miles into the drive home with 60 miles to go, my
RPMs were staying right at 4 RPMs and 4th gear
when doing 80 mph. Normally my car would shift
(it’s automatic but I have a gauge that tells me
what gear I’m in) to 5th gear. I started experiencing
this right after we got over a mountain pass where
my car tends to struggle. I watched it stay in 4th
gear and stay at 4 RPMs and then I decided to pull
off because my car started to sound like the
Jetsons space ship. I had it off for a few minutes
and it started up again but still sounding bad. I got
back into the freeway with hazards on and keeping
speed down to make it to a gas station which was
the next exit and my husband opened my hood
and couldn’t see anything wrong but that my car
was leaking coolant which I had just filled up
before our trip, and it was a lot. So we got it towed
home and have a mechanic who we dropped it off
at who will look at it sometime this week but isn’t
probably going to look at it anytime soon. I just
want to know if anyone knows what it could
possibly be? I had no car lights on for the engine
or oil, nothing. So I am really in the dark on this. If
it’s an engine problem, which my husband has
blown his truck engine before and thinks this might
be the case, that will cost more than my car is
worth and we will have to find another vehicle for
me. Just wanting to know if anyone has any idea
what this might be so I know if
I should be looking at other vehicles or whatnot.
Thanks!