Help! Could this be Austism?

couple weeks ago, my 13-month-old daughter had a vocabulary of 15 words. Over the last couple weeks, she is speaking less and less. I've read that regression in language can be a sign of Autism...but I don't know that she's really showing other signs of it, and I'm not sure that she's old enough to be showing signs anyway. She has never been an overly social child, but she is very friendly and affectionate towards people she knows as a general rule. I have always wondered a little about her concentration, which seems to me to be unusually good in a child her age. She will concentrate on something (blocks, pulling books off a shelf and trying to put them back) that catches her interest to the point where she doesn't seem to hear or see anything else. But everyone (including her ped.) tells me that she is fine. This apparent regression in language is extremely new though, and I don't know if it's too early to worry about it or if she's just spoiled or if something is wrong. For example, she used to say 'Baba' when she was thirsty, but now she's just fussing 'til I figure it out, and she used to say 'Boo' when she wanted her security blanket, but now she just points and babbles if it's out of reach. (She will go get it herself, if she can). I'm really worried, and I really need some advice. I'll take her to the doctor again if she keeps doing this, of course, but I need some advice in the mean time.