Why are her periods starting later and becoming lighter each month?

Let me say first off,

not all periods will be the same all the time.

There are many of factors that can cause a period to be lighter and or shorter even skipped

Stress

Diet (such as eating less )

Hormonal Problems

Hormonal Birth Control such as : Pill, patch, ring, shot, and hormonal implants

Worry and anxiety of becoming pregnant (this would count as stress)

Weight loss or gain

Illness, recovering from surgery

Age- over time our periods will regulate themselves, but when first starting out it can skip ever so often.

Excessive exercise - great for your body, but if you over do it, it can really throw your body out of whack.

Pregnancy

If there is a concern about pregnancy I would do the following

Take a home pregnancy test, follow the directions step by step (I would use first morning urine, its most concentrated)

Or Go to a health care provider or a Lab and conduct a blood pregnancy test to confirm you are not pregnant.

NOTE: Be sure to wait AT LEAST 1 week from your irregular or skipped period to take the test (whether blood or urine) so that you prevent false negatives or positives...If you're still unsure, retake your test one week later.

Let me also say here while we are on the topic PREGNANCY SYMPTOMS ARE NOT A VALID SOURCE OF DIAGNOSING PREGNANCY!!! When it's acutally PMS symptoms, these can be confused sometimes as pregnancy symptoms...The Only difference between the two, it that it does not make your test positive.

So, if you had unprotected sex or your method of birth control failed, Or if sperm came into contact with your genitals there maybe a risk. If you did have protected sex and your method had no likely failure but you are still unsure, it's okay to test and take your mind off things. I highly recommend this to any worrying woman.

Sometimes bleeding in between cycles can pose a problem.

IF it persists you might want to go in for a STD screening as well as for your yearly Exam to see what the problem might be.

Otherwise, periods are not always the same, but if you're still unsure call your OBGYN and see what he/ she thinks, and get tested.. Arm yourself with knowledge and power.