Thanks for the query.
As your urine pregnancy
test was negative after one week of missed period, the possibility of pregnancy is less.
If you want to be sure about the result better to go for blood test
If the test comes negative, the possibility of pregnancy is very less.
The intermenstrual spotting and irregular periods
you are having could be due to hormonal
This can occur due to stress and strain, weight changes etc.
The symptoms can change from cycle to cycle depending on the severity of hormonal imbalance.
So you once consult gynecologist, get examined.
And go for ultrasound
if she suggests.
Then you can take tablets to induce withdrawal bleeding with your doctor's advice.
If the irregularity repeats, you can use hormonal pills for few months.
This will help in regularization of the cycles.