Is it possible that she has a very 'full plate' hence very little free time....does she have a full time job in addition to her other obligations?
These days, most (working) couples jointly manage their household, two jobs, the bills, their children and if there's any time left, a somewhat limited social life. Most weekends are for running errands, cleaning house, repairs/maintenance and laundry....did I mention children with outside activities that require transport and parental participation? Most have limited time with family because of their very busy schedules.
If your daughter needs help with babysitting or you're feeling 'used', it's because you're probably the only people she feels close enough and comfortable enough with to 'use'.
Without a crystal ball, it's hard to tell if something more serious is happening in your daughter's life. You also need to ask yourselves if you had a legitimate prior engagement or other valid reason why you couldn't babysit or was it a matter of pride that led you and/or H to overthink the rest?
Either way, the healthiest approach going forward, will be to assume good faith on her part, keep honest lines of communication open and always be unconditionally supportive.