A common cause of missed or irregular periods is, indeed, pregnancy. If you've taken a pregnancy test this month (since you missed your period in May) and it's negative then at least you can rule that out.
Next, you need to talk to your doctor and let them know about having only one period so far this year. There are many, many things that could be causing your irregular periods and nearly all of them are pretty innocent (but frustrating, because we kind of come to depend on that monthly period) so let's look at some of those possible causes:
- Peri-menopause or menopause: This is just when your body stops having regular menstrual cycles due to age. Peri-menopause comes first and can begin in your 30's or 40's.
- A thyroid condition: If your thyroid is a bit out-of-whack this can cause late or skipped periods. A simple blood test can help your doctor diagnose it and the treatment is also simple (medications, usually).
- Polycycstic Ovary Syndome (PCOS): This is a hormone imbalance that messes with ovulation and is treatable with your doctor's help.