Is it i was going to fell down or i was going to fall down?

You would use "fall" because "to fall" is the infinitive and you use the uninflected version.

"Fell" is the past tense of "fall," but you are not using past tense here. "Fell" can also be a transitive verb meaning to cause (something else) to fall. If you chop down a tree, you fell the tree, but the tree falls.