It's not unusual to miss periods at age 45, even if you haven't missed any before. It's part of the gradual process that eventually leads to menopause (menopause, the cessation of periods, may still be years away).
If by water infection, you mean a bladder infection, I would see your doctor about that. Mention the missed periods and the light bleeding.
If the light bleeding persists, it can be treated with progesterone pills.
Your doctor may wish to do an exam to be sure everything else seems normal.
If pregnancy is a possibility, then a pregnancy test would be a good idea. As long as the bleeding isn't heavy, it's usually possible to get a clean urine, possibly by putting in a tampon.
As long as you've been having regular periods up to now, and you've had normal Pap smears, the light bleeding is unlikely to be from anything serious. In case you are pregnant, it could be a sign of miscarriage. Best wishes.