I haven't dealt with full on ppd, but I have had days after each of the kids, especially in the first week or two, where I was really emotional and cried super easily! So far this time I have cried when I couldn't get tickets to a concert online, cried when I found out some friends of ours tried to get them for us too, cried when Franklin the turtle's friends were nice to him (this is a kids cartoon!), cried when my hubby realized I was tired and offered to finish up dinner with the kids so I could go lay down, cried over stories people have told me, and laughed until I cried - then I couldn't stop crying for a few minutes, even though it was still funny to me! I think getting emotional or crying easily is pretty normal. Once I start crying, I cry really easily for a little while too!
If you can't stop crying still, can you get someone there for some support? A hot shower, a nap, or a meal with fresh fruits, veggies and a good source of iron might help a lot. Prenatal Vitamins and Omega 3's can also help pick you back up a bit.
If it keeps up, gets worse, if you are really upset or really sad, or having thoughts about harming yourself or little one make sure you call someone right away! Even your midwife, OB, nurse, etc. can be a good person to help you sort through things! It can be hard to make a call like this if you are already feeling really down, but it is important to get some help if you need it, and nothing to feel ashamed about!