I agree that a speech evaluation would be helpful here. Your primary care doctor should know someone to refer you to. Speech delays are not uncommon and sometimes it is as simple as AnnieBrooke says in that they aren't talking and wake up and use full sentences. It is a good sign that she understands you when you tell her something and can follow a direction. However, speech issues also can be tied into other things. If a child has some motor issues that are even mild in nature, articulation of words can be difficult. Where you see things next would be in the area of fine motor ability. That includes doing any unfamiliar fine motor activity or using a pencil to using scissors, etc. My son has sensory integration disorder and the first signs that a preschool teacher noticed were garbled speech and frequent tripping. This was because my son has some issues with motor planning which controls/organizes our processes to say the words and carry out motor activities.
The other reason why it is important to address speech early on is because having issues with speech can hinder social growth. It is hard to play with friends if you can't communicate with them. At almost 3, she'll be coming out of parrellel play soon and moving onto play with peers.
So, I'd take her to a speech specialist and have her assessed. You can also do some mouth exercises--------- turn on music and you two make funny faces and then freeze when you stop the music. Have her move her tongue side to side, trace her lower lip, stick it out and pull it back in fast. Eventually move to moving the tongue up and down (up is really hard for kids your daughter's age). Speak slowly to her and have her look at your mouth. I also like the CD "Speechercize". You can blow bubbles, blow a cotton ball across the table with a straw cut in half. Chew fruity treats or anything chewy. All exercise those mouth muscles you need for speaking.
good luck
I'd get her evaluated. Sometimes delayed speech suddenly just "clicks," but starting early is a good idea if you do need to get her into speech therapy or other assistance. My nephew went through this and did go in with a speech specialist weekly for a while, and it really helped.