If she is drinking from a sippy cup, I wouldn't worry about actually using a bottle - many babies get kind of fed up with bottles at that age, and if she's able to use a sippy cup - there is not reason to keep her from using that. You may also consider that the sippy cup flows faster, and breastfeeding is much nicer than using a bottle - so, she may have developed a preference for breast and/or faster flow. Babies breastfeed rather efficiently (even at night), and will take more from the breast as they get older even with shorter feeding periods.
Just watch her weight/health and wet/dirty diapers and you can tell if she is getting enough. My daughter is 1yr and has almost been exclusively breastfed - she nurses a lot in the evenings and at night, and as soon as I started working again, she went from sleeping 10 straight hours, to waking almost every hour. Babies want to breastfeed - its more than just nutrition - so, they often prefer the breast over bottle, it's actually a rarer situation for them to prefer the bottle over the breast.
What if you express milk and put it in a bottle/sippy instead of forumla?
It doesn't take much to change their routine - the simple adjustment over the weekend could have changed her, and with more breastfeeding and mommy-closeness, she has discovered that this is more fun and she has changed to prefer that better. I have heard of many breastfed babies who will eat only about 8oz while mommy is gone, and then breastfeed a lot in the evenings and night to make up for it. This is very "normal" for a baby! There is really no right way a baby should be - and you also have to understand that breastfed babies are very different than formula-fed babies - they love their mommy time because that means comfort and love in addition to nutrition.
I would not worry as long as she seems healthy and is gaining appropriately with wet and dirty diapers (particularly wet diapers).