You should be referred to a gynae for further evaluation and diagnosis.
Irregular periods are often caused by hormonal imbalances. If issues with the thyroid are ruled out (and this can be determined through blood tests) then another common culprit is a condition called polycystic ovaries. This can and does affect ovulation and can cause irregular periods.
The day 5 test you had done should have been done two or three days earlier; the standard test in that regard is called a Day 3 test. This measures LH (luteinising hormone) and FSH (follicle stimulation hormone). LH and FSH are important hormones as they kickstart ovulation; if these are out of sync then ovualtion won;t happen. Both levels need to be COMPARED against one another; if this was not done the test result is meaningless. My guess for what it is worth is that the levels were okay primarily because the LH and FSH levels were not compared.
Do you remember offhand what your LH and FSH levels were?. They should ideally be the same; a level of LH which is higher than FSH is one possible indicator of PCO.
The day 21 test measures progesterone and here they like to see a result of 30 or more.
As your cycles are so irregular the blood test can be done according to calender days i.e the 3rd and 21st of each month. It is hihgly likely that you are not ovulating regularly if at all.
You should in any event be referred to a gynae at a hospital's subfertility unit. The GP should refer you asap and do not take no for an answer. These are not problems that should be messed around with by GPs.
Your bad period pains as well are worthy also of the attention of the gynae - endometriosis can cause severe period pains. However this won't be picked up on a blood test, a keyhole surgery op called a laparoscopy is how it is detected.
You need a gynae referral and a diagnosis.
You will also need to be persistant in order to get answers.
Attila