Her memory may be developing, and she may start recalling some parts of her babyhood and how good it felt to be cared for as an infant. Plus, pretending to be a baby for a while can be fun. It certainly has its benefits, like receiving your undivided attention and snuggling in your arms.
One explanation could be that your child's reacting to change by retreating to a safer and more secure past. Many toddlers regress when there's been a shift in their routine. Has your child recently acquired a new sibling or started preschool for the first time? Or are you planning a move, or having problems with your partner? Any of these life changes can upset a toddler's budding sense of control, triggering babyish behavior. If so, try to alleviate the tension she feels with lots of hugs, attention, and time with you. When things settle down, she will, too.
Your toddler may also be trying to tell you that she's stressed out by trying to be a "big girl" before she's ready. If she has previously ditched the bottle and now insists on one when she's at home, it could be a sign that she's overwhelmed by trying to be independent too quickly. Try scaling back your expectations. Instead of requiring her to clean up her mess herself, for example, help her straighten her room. If she's having trouble dressing herself, offer to help out until she feels comfortable. When she asks for a bottle, try distracting her to see if she forgets her request. If she persists, you may want to let her have it once in a while; but instead of making a big fuss about how disappointed you are that she's back to using bottles, praise her when she does use a cup. If you focus on how great it is that she's using her "big-kid" skills instead of how she's acting like a baby again, she'll probably let go of her bottle fixation soon.
But if her attachment to the bottle seems ferociously renewed and shows no signs of dissipating, and is accompanied by other intense and constant baby behavior, such as wanting to be carried all the time, consult your pediatrician to see whether something else might be going on.
If your toddler's regression seems primarily limited to asking for lots of attention — such as wanting to sit on your lap more often or asking you to watch her while she draws — give her the love and attention she craves. Children are entitled to ask for what they need, and sometimes what they need is simply more of you. Toddlers are changing so quickly that they sometimes have to fill up on parental love and affection to shore them up for more of the maturing they do every day. Think of them as travelers preparing for a long trek; throughout their journey through childhood, they need to go to the well (you) from time to time and fill up on the confidence and love they need to do the work of growing up.
You may find that the trickiest part of dealing with your toddler's regression is handling the episodes in front of others. If your child throws a fit and asks for a bottle in front of Grandma and Grandpa, who think she's too old for it, try to make as little of the request as possible. A simple remark that everybody needs a break from being grown up can help put her actions into perspective for those around you.