word meaning anticipatory disappointment

letdown 2. A disappointment
The cancellation of the game was a real letdown.

If you don't yet know you're going to suffer a letdown, but you're increasingly fearing you might, you could say you're having qualms/misgivings, or you're apprehensive.

foreboding

A feeling that something bad will happen; fearful apprehension

This is from Urban Dictionary so you can't use it in formal communication but it seems to capture what you were getting at.

Anticipointment
a gut wrenching emotion felt deep in the belly of a person unrealistically looking forward to an upcoming event immediately after the reality of extreme disapointment has replaced the anticipated happiness

There is also:

dismay
dread
unease

but I personally prefer anticipointment.

You got an ominous feeling in your stomach.

giving the impression that something bad or unpleasant is going to happen; threatening; inauspicious.

You could also go with dark, unpromising, or unfavorable. But I think ominous works best here.