How to calculate the number of days between an event and the previous one for each individual

Question

mydata<-data.frame(
  ID=c(1,1,1,1,2,2,2,3,3,3),
  day=c(2,4,7,22,8,9,22,4,7,17))

In this dataset, I have 3 patients who have 3 or 4 follow-up points. I want to calculate the number of days between each follow-up point and the previous one. I need to have such a table, the difference for the first row after sorting days un increasing way, should be 0

  ID day diff
1   1   2    0
2   1   4    2
3   1   7    3
4   1  22   15
5   2   8    0
6   2   9    1
7   2  22   13
8   3   4    0
9   3   7    3
10  3  17   10

Thanks for your help