Hi, S.. Well, not knowing your daughter personally, my guess is that she's seeking attention. This shouldn't turn into a big issue; make sure you deal with it calmly.
First, I'm guessing that she's missing the nurturing attention she got as a baby and wants to relive the nurturing as well as the pleasure of sucking on the bottle (she's only 2 yrs old, still halfway a baby). Has anything happenned in the past couple of weeks or months that has given you less time to spend with her? Is she demanding the bottle at night to get her to sleep? She might be having some anxiety about being alone in her room or going to sleep at night. This is also a time when children can start getting nightmares. If she needs the bottle again to sleep, maybe she's a little scared of her room and uses the bottle to comfort herself.
I think it's more likely that she's craving more attention and doing it this way because it's sort of negative to regress, to go back to an earlier stage. She probably knows or senses that she has your disapproval for this, and when children seek out negative attention, it usually means they need more positive attention but can't seem to get it.
I would make sure I was giving her more attention about other things (other than the bottle) at other times than when she's regressing to the bottle...and to place no special attention on her using the bottle. In other words, give her cuddling and playtime and smiles and other attention when she's NOT sucking on the bottle -- and don't give her any when she IS sucking on the bottle.
If she's doing this to get attention, and you give her the attention she craves for doing other things, she'll stop messing with the bottle or greatly decrease it.
Another factor could be that the sippy cup sort of leaks on her when she's drinking, while the bottle does not. Maybe she realizes this and is using the bottle to avoid getting cold liquid on her. It's possible.
Well, I hope you can get her off the bottle once again. It's not harmful unless she starts sucking it all the time or lets it go on for a very long time. My ex-roommate had a daughter who decided she had to have a bottle to get to sleep even when she was 6 or 7. If she didn't go to bed with her bottle, she had a fit and wouldn't sleep. Her mom made a big deal out of it and gave her lots of attention around the bottle ritual. I think the little girl finally got sick of it and lost her bottle one day -- she sometimes walked around with it hanging out of her mouth even at age 7, in front of all her friends, too -- and just quietly went to sleep. But she didn't have the words to tell us that she was done with the bottle ritual -- she just lost the bottle.
It was unnecessary drama for the whole family, when she could have had healthy attention. To this day (she's 12) she's addicted to drama and causes it all the time, just to get attention.
Anyway, these are my thoughts. I hope everything works out un-dramatically for you!
Peace,
Syl