Before 2nd child i used to catch virus twice a month.i was truly shocked how and was thinking to see an immunologist as well.however after having 2nd kid i hardly ever get sick and its so surprising. pregnancy fixed low immunity?how exactly?

Most likely your immune system is no different and no more strong than before. A more likely explanation is that over time you have been exposed to more common respiratory viruses and their variants, and therefore have partial or complete immunity to them. Your first child probably caught many viral variants from day care and other kids, and you're now immune to many of them.