Bonjour Jenny,

When you say "J'y suis tombé(e)", you're saying "I fell (in) there".
The adverbial pronoun "y" here is used to replace a place that could have been mentioned previously. I agree that out of context, it sounds a bit weird!

If you meant "I fell", you could of course say "Je suis tombé(e)".

In our lesson, we're using the example "J'y suis allé(e)" (I went there). In that case, you couldn't just say "Je suis allé(e)" (I went), as in French the verb "aller" always needs a mentioned location (i.e. to go where?).

I hope that's helpful!

Aurélie