'Baby Blues' are perfectly normal some time in the first few weeks after the birth. It also takes a long time for your hormones to settle after giving birth so bouts of tearfullness is totally ok. I used to cry every time I fed with my first, something to do with hormone surges I guess.
So many women worry that they have PND (post natal depression) just because they get fits of the tears but for PND you need to look for more than just tearfullness, such as feeling very depressed, not interested in baby, not getting washed and dressed at all in the day etc.
PND doesn't nornally start to show until about 6 weeks but if you do feel worried at all just speak to your midwife/ HV or GP as it is easily treated now and nothing to be worried about.
Much love