How do I get my 13 month old to drink more?

She is still breastfed but only 3-4 times a day for less than 5 minutes each time. We always have a cup of water available for her and offer her 2% milk during meals but she drinks less than 3oz of each throughout the day. She keeps getting constipated because of not drinking enough. If feel so bad because the past 2 days it was bad enough that she cried when she was trying to poop. I’ve tried offering a little bit of juice but she won’t drink it.

Any advice for getting her to drink more so this stops happening?