I'm 30 yo with regular cycles of 32 days. However, my last cycle was 42 days, and this one is already on 45. What could cause long menstrual cycles? I took a pregnancy test and it was negative.

Endocrine. There are several causes for an irregular menstrual cycle pattern. It is best to be evaluated. Some of the causes are hormonal and can include thyroid dysfunction, premature ovarian failure, pcos, hyperprolactinemia. A change in your general health can also cause a change in your menstrual cycle. Stress can also contribute.
OVULATION PROBLEMS. If you are trying to conceive, you should see if tou are ovulating. You can do this on your own by doing basal body temperatures, or use urine ovulation tests, which may be a bit pricier. You could also see your gyn. Doctor for evaluation. You could be ovulation normally, but on an irregular basis. Many women do!
Stress. The brain and ovary are linked. If there have been changes in your diet, sleep, exercise, or life experience producing chronic stress, that can affect your menstrual cycle. Listen to your body. Move in a direction of improved health. Avoid alcohol, tobacco, processed food, eat healthy, get good essential fatty acids (olive oil, flaxseeds, butter from grass fed beef), don't over-exercise, destress.