What could be wrong if 29 and you had a regular period on the 4th - 8th an irregular again 24th 26th one-day next month 20th but HPT is negative?

Several reasons could explain a negative urine pregnancy test despite irregular periods:

1. Early Pregnancy: The pregnancy hormone (human chorionic gonadotropin or hCG) may not have reached detectable levels in your urine yet. If you think you might be pregnant, repeat the test in a few days or try a blood pregnancy test, which can detect hCG earlier.

2. Ovulation Timing: If you have irregular periods, your ovulation could be unpredictable, leading to fluctuations in your menstrual cycle. The irregular spotting you experienced on the 24th-26th and 20th may be related to hormonal changes or implantation bleeding (if you are indeed pregnant).

3. Hormonal Imbalances: Conditions like polycystic ovary syndrome (PCOS), thyroid issues, or high levels of prolactin can affect your menstrual cycle, leading to irregular periods and potential difficulty getting pregnant.

4. Medication Interference: Certain medications, such as birth control pills, hormone replacement therapy, or some antidepressants, can interfere with regular menstrual cycles. They may disrupt ovulation or affect hormone levels, causing irregular bleeding.

5. Stress or Weight Changes: Emotional stress or significant weight fluctuations can influence hormonal balance and interfere with menstrual regularity. This can result in infrequent or atypical periods.

6. Premature Ovarian Insufficiency (POI): A condition where the ovaries stop working normally before age 40 can cause irregular periods and reduced fertility. If you're over 40 and noticing irregular menstrual patterns, POI should be considered.

If you're still experiencing concerns or want to rule out any underlying factors, it's recommended to consult a healthcare professional for further evaluation, advice, and testing if necessary. They can provide personalized guidance based on your specific situation and provide further support if additional investigations are required.