When Should I Expect My Next Period?

Add 28 days to the day you finished your period, and then give or take three days. That should give you a range of when to expect it, although not everyone is completely regular.

I had my period on 20th January and finished on the 25th January. When do I expect my next period?

Most women get their period about every 28 days; if you haven't been keep track or are just starting then best to assume every 28 days, and then watch what happens.  Anything between every 22 and every 45 days is within the relatively normal range.

Otherwise, you need to keep track for 3-6 months of what dates your periods started on.  Then you count the number of days between the start of each period.  After you have 4 or more cycles to measure between, you will probably find a fairly consistent pattern: Say every 28-29 days.  

Online there are several calendars that will help you quickly see when you can expect each period to come in the future (see examples).

Some women do have irregular periods; 22 days, then 29 days, then 45 days, then 36 days, etc.  This isn't a problem by itself; it's just how some women are.  But it can make it tricky if trying for a baby.

I had my period last month on the 7th when should I expect the next one??

The history which you are giving indicates that you are suffering from polymenorrhea. This is a condition in which cycle length is less than 22 days. The normal cycle length range is 26-32 days. Above or below this range This is due to short secretary phase and progesterone deficiency. Can be treated by taking progesterone in the second half of the cycle.