What Causes Delayed Period While UPT Is Negative?

hai doctor , i am banu 28 years old , married and trying to conceive for last 6 months. My period cycle is 36 or 37. Now i have missed period 44 days, i had taken hpt test on 40th day but result was negative. i have watery discharge and backpain. alternativedays i feel very tired. pergnant symptoms or not? When i take hpt test again?