The same happened with me when I had my son 6 years ago. I was usually about a 10/12 and after I had him I lost a lot of weight probably the same as you and then I began to blow up out of control, I was so depressed.
I then began putting miles on my stroller, he like the outdoors and so did I. When I took him to the park instead of sitting I walked some time twice a day. I did this for about 20min. I never skipped breakfast, but I cut it to a half of a bagel and something hot. Tea w/lemon worked best for me because if drink coffee I drank it w/cream and sugar.
I drink water instead of soda or juice.
Ate a regular size lunch and a half of my dinner.
W/e was dedicated to my husband and son so it never watched so much during then.
I continue to this day doing the same thing and have been in a size 8 for the last four years maybe.
If I haven't had time to walk I cut down on my portion sizes.
If I’ve been bad and gained a few I modify it but I don't stray from this pattern.
And with God's help it works! I should add that before beginning and even during this I asked God for his guidance to help me with my motivation, my choices and my endurance.
M. B