What Causes Irregular Periods While UPT Is Negative?

i m 29 yrs old, newly married 4months back. before marriage i have a regular period. but starting 2 months of my married life i have irregular period, i.e. in 12 days, after that again i have regular period. but in this month again i have irregular period. i have done pregnency test. but the result was negative. is there any problem? in this month i lost 2 kg weight. my weight is now 48 kg