Started solids, baby is constipated

Hi, my 5 month old was drinking so many ounces per day so we decided to start solids a little early. She did oatmeal for a couple of days and then we gave her some avocado. Well it’s been a week now and since she has had the avocado she has been straining so hard to poop. Her poop is also now solid. To make it worse, if she starts to poop right after drinking a bottle she spits up while straining. HELPPPP how can I make her poop softer and make this not such a horrible thing?

I was thinking prune purée for one meal?