Wow! I can understand your concern but please, please, please - listen to what everyone is saying below. Hard not to worry, but please don't. My daughter had all of the same things your little one has and she is JUST FINE! I went through the panic stage and didn't need to...wasted energy on negative stuff.
On the way home from the hospital (she was 4 days old, I had a c-section) we stopped at the dr. office (we were told by mistake, by a new receptionist, that we had to make an appt. during her first 4 days). Anyway...we hadn't even had her home yet and they found ***4*** murmurs. We found out that many times in c-section babies, the heart chambers don't fully close up because the babies don't have those last pushes through the birth canal to solidify...who knew?
I fell apart. 4 day old baby...4 holes in her heart. That's what a murmur is - the heart "seams" just haven't closed up. They most often do with time and growth. I whole-heartedly believe that your 12 month old will grow out of this as he grows up and his body continues to grow and change.
Weight gain. Maggie was at the 95th percentile and all of a sudden dropped to the 50th. She even had some months when she had dropped a bit in weight. My dr. had me come back in every 2 weeks for weight checks. Again, a nervous wreck. Bad mom thoughts...am I not feeding enough, is she not getting enough nutrients, etc. etc. etc. Again...guess what? Is your son becoming more active? He is 12 months old. He is probably cruising more than he had, he is probably getting the crawling thing down. Is he beginning to walk? Becoming more active burns calories! That is what is going on most likely. We started to add good fat foods in Maggie's diet. She ate a lot of avocados and we would put olive oil on her pasta, etc.
Again, please don't worry as these things (although they don't tell you when they give you the things that are "wrong" with your child) are all very normal.
Maggie just turned two and she only has one murmur left and they figure that it will close up in the next few years. And for weight, she has stayed at the 50% at her visits and has continued to grow.
Be well A., take care of yourself, and KNOW that you are doing an awesome job as a mom. Remember that nurses and doctors see patients ALL day and see the same things ALL day. They are routine to them and HORRIFYING for us to hear because it is the UNKNOWN.
Good luck and get someone to watch your little one while you take a bath and let some peace set in. Don't make the same mistakes of over-the-top worry, take away the next couple days you have with your little one!
J.