I had the same problem with my oldest daughter when I married. She was 7 at the time and a very happy kid. Then all of a sudden...she started wetting the bed. She even wet her pants at school once.
I sat down and had a good talk with her and found out that the situation of the marriage and me expecting a baby was just too much for her at the time. Being so young, she just didn't know how to express herself.
I ended up spending even more time with her (one on one), and making sure the 3 of us (she, my husband, and I) sat down once a week and talked to one another about how we were doing with eachother. Or we watched movies together (her choice).
I also made sure that when family and friends would visit, that hey made sure to reference my oldest daughter "before" they began talking about the arrival of the baby. Therefore she didn't feel as though the baby was taking her place.
For us...these little things helped. By the time the baby came, my daughter was no longer wetting the bed, and was very involved in the day to day caring of her little sister.
Perhaps your 10 year old doesn't quite know how to express herself either. Perhaps the divorce, the boyfriend, and the changing of her world is just too much for her to handle right now. She may just need a little more emotional security from you ensuring her that you won't leave her and that your boyfriend won't take her place in your heart.
I would suggest a "Mommy & Me" date with her...filled with things she loves to do. It can even be at home doing nails, eating Mc Donalds and watching a movie. Just as long as it's only the two of you. Then invite her into a comforting loving conversation about what ever it is that's bothering her.
Hope this helps a little.