Infection for 6 month baby. Please help!

Hi my daughter is 6 months old. Recently I started solids (steamed carrot) and as she showed constipation issues I gave one piece of smashed banana also I gave iron drops same day. The next days onwards she started vomitting whenever I breast feed her. Consulted paed and she is on vomikind and cefixime antibiotics. After taking vomikind the vomit is stopped but as soon as I breastfeed her she is pooping watery (looks like diahorea). I have two concerns Her stool is pale yellow colour which bothers me Whenever she rolls over, or whenever we hold her upright , she is arching her back and she is not able to hold head straight. She is arching backwards and forcefully she is trying to keep her head straight. This head arching is happening only after this infection. Before that she can hold her head stiff. I'm very much worried about both the things. Please help