my son is way to tiny 😔😔

I was just wondering has anyone else gone through this ... this is my 2nd pregnancy first one was great I went past my due date actually was over weight and my daughter weighed 8 lbs. 4 oz. .... this time around not the same tho ... I'm finding I am quite smaller than I was first time around but figured because he's a boy ... went to my a appointment The other day & there saying my son is very small I'm 30 weeks four days & honestly all I've been doing is crying since I found out I'm so scared for him & all I can do is I pray .. I asked what could I do to help his weight gain they said nothing ... I don't understand tho I'm gaining so much weight why isn't he gaining ??!! Someone please advice ? Or any similar situations I'm driving myself crazy 😔😔 . Could it be I have a2 year old & im always carrying her screaming chasing her etc ? Like please I need help 😩😩