I should be menstruating on 1st of every month and its 6th today I did not have my periods .I had sex but we used condoms can I be pregnant.. cervical mucous is slippery and can be stretched into thin threads this shows estrogen I still have doubt..

No woman can really say she 'should' menstruate on the first of the month. For one thing, months have different numbers of days, and for another, it is very rare to be consistent over a long period unless you are on the pill.
Condoms can have tiny holes, can spill over, and so on. They have a failure rate that is pretty high.
Mucous and missed periods and any symptoms you can think of prove nothing. Only a test proves if you are pregnant.

I will agree, you have a certain number day cycle, some months have 28 days, some 31 days, so your period would vary from month to month just because of different number of days.
From planned parenthood web site, condoms have a 2 to 18 percent failure rate

But if you had sex yes of course you could be pregnant.

First thing is to figure when the period was actually due, do you actually track the day ?
How many day cycle are you on?

But once you know you are really late, you wait two weeks past due date of period and then test using the first urine of the morning as soon as you wake up