It's so funny because you could have written my story right now (and did). I go back to work tomorrow and my DD is doing the same thing yours is doing. She doesn't have a sloppy latch, but she is refusing to nurse at different times and has dropped some nursing sessions. We don't co-sleep so some days she only nurses well 2-3 times the whole day! I've been able to mostly maintain my supply, however, she eats quite a bit for a 13 mo. old and she drinks a lot of water so I don't know how much bm she is actually getting. I stressed over this a lot but my thought now is that I will keep offering and as long as she nurses even a little I will have milk for her. I think you will be fine as long as you keep offering. I have heard a lot of moms say that around this time (11-14 or 15 mo.) babies get very active and forget about nursing... so, just keep offering your breasts, keep a sippy cup of water around, and give her lots of healthy foods to choose from.
One other thing - I think you might find it hard to get her to nurse lying down and staying still. You might have to find some other ways that work for both of you - nursing her in your lap, while she's standing up, kneeling over her, side nursing, nursing her in her sleep, etc.