Want to know if I am pregnant or not.

Age:27+ have a baby boy of 3 yrs old, I am sexually active with my hubby but I use condomes all time I have sex.My LPD was 10th Dec, 2019. I have taken pregnancy tests 3 times, it became negative, i do not have any pregnancy symptoms as such like nausea, headache, having excessive smells etc. I want to know the reason... In the mean time, I had minor lower abdomen cramps for 3 times approx. However periods didn't come. Please tell me the reason of it. Can I take another pregnancy test? And I do not have any pre periodical symptoms like backpain, painfull nipples or tender breasts etc. P.S earlier I had a 28 to 31 days cycle and it was pretty much regular since after 4 months of babybirth, I am facing this kind of irregulerities and in the month of september and october, 2019, I got that perfect periods cycles happened and suddenly it happened with me...and I have faced 1 month delayed periods maximum, but this is of 3 months delayed periods this' very new to me and I am worried.... PA