After battling in Smash Bros so did Robin start to study Lucario in his natural habitat. She spent months close by and studied, but it seems like Lucario noticed her during that time (or always knew of her considering how lucarios can sense aura in all living things), and they seem to be on friendly terms. They're even happy with each others.
No idea about her big belly though...
Maybe she just found time to date someone and it went a bit far?
Robin and Lucario (c) Nintendo