Hey there BZilla. I will preface this by saying I am NOT an MD, and this is just what I know from some hormone study...You should definitely check with a doctor if at all possible, because the loss of a period can be more than something obvious. However, these are some questions to ask yourself...
Amenorrhea can occur because of weight loss, but that's usually extreme weight loss in a short period of time - 87 lbs. in 2 years is not extreme. It can also occur due to excessive working out. Female athletes often get amenorrhea, though that is because they are working out HARD for hours a day (usually 4-6+ hours). Have you been working out that hard?
Also, have you completely lost your cycle, or are you simply having irregular or very light bleeding? Do you have any "symptoms" of your cycle (such as any PMS symptoms you would have before your cycle, but simply without the menstruation?) Have you checked your cycle/ovulation with an ovulation checker (which you can purchase at a CVS or similar store) to see if you are still ovulating on time? Do you have any other symptoms such as pain during any point in your cycle? Have you ever been diagnosed with a thyroid problem?
The loss of a period can indicate a lot of medical problems, or it may indicate that you're working yourself out very, very hard. You can even lose your menstruation due to stress. It can also change from going on/off birth control -- I know I've switched the amount of hormone in my birth control pill and lost my period for two months before, though not for something like 6 months. I know that Depro Provera as a birth control method can often cause ceasing of the menstrual cycle.
Female, 28 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/3/12 = 164.9 (net: -50.3 lbs.)
Current mini-goal: 160 lbs.
Next mini-goal: 150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."