Post pregnancy workout. Help please

Leah • Pregnant with #2 have a 6yr old son from previous relationship. Work full time at the moment as a activities coordinator
Hello, I had my son a week ago and want to get back in the gym. I stopped working out (except swimming) at 19 weeks pregnant after hurting my back and knee. 
Now I don't know how/when to start back up. I feel physically good to go my back pain has gone completely and my knee injury was strengthened during the swimming (torn tendon). A bit of advise would be really helpful. 
My main work out before was HIIT but I don't know if that's the right place to start