Hi..Thanks for the query..
First of all taking too many i pills not good..
The fertile period for conception for a 28 day regular periods is 
between 9-19 days of cycle.. The remaining days are considered safe 
for intercourse without running the risk of 
pregnancy..
If you have intercourse in the above said days (safe ) you don't 
have to take even i pills provided your cycles are regular and your 
calculations are proper..
One more method to calculate safe period is going by the quality of 
discharge..
Intercourse is safe during dry days. Intercourse during wet days 
(slimy transparent discharge ) has the risk of pregnancy..
The reason for irregular and scanty periods may be taking too many 
pills..
Also improve your general health so as to get regular periods..
Hope this information helps you .. Take care..