My son has just started to go through the same thing (he is almost 16months). Before he loved the bath and now he will come into the bathrom and try to run out before we put him in. Here are some things that we have tried and now (2weeks later) he likes it again and doesn't want out.
-we had a shower/bath with him 1 time and made it really fun
-let him bring his favourite balls (little soccer ball and basketball) into the tub with him and created a throwing into the cup game
-had a bubble bath a few times
-cleared all toys out 2 nights and he just swam around on his belly - sometimes too many toys restricts their movement
-kept him in the bath and told him that he needed to get cleaned off and then proceeded to talk to him about cleaning all the parts of the body and why it was so important - sometimes this happened through the crying
-kept the tap on a little bit and made a game of filling up cups and dumping them on his toes/head/my hands
-a good splashing game
-getting him in there quickly - undressing him in the living room, having the bath ready and just taking off the diaper in the bathroom - preparing him but having it all ready before we went in
We trired to think of so many things to make it enjoyable for him again. We also realized that this started to happen after he had been bathed in the sink at my mom's house a few times, so there might have been some confusion there for him
He really enjoys the sink bath though - even though he is 16months and barely fits - he really enjoys stepping back and forth from one sink to the next and also the running water on his body - I think this fun in the sink made him really wonder why he had to get in the tub at our house. My moms bathroom is under renovation and we are there a lot with him.
I would just try new things but I would continue to bath her just as often as you did before. Some of my friends stopped bathing as much when there lo started to fuss about it and now they have a real problem. We are just going to continue to bath him everynight and try to change it up a little.
I hope some of these ideas help