spotting before period gets heavy, normal?

So recently I've been noticing my period has changed. I used to start my period and it would be normal not heavy Day 1 and then day 2 would be my cramps and really heavy bleeding into day 3 and after day 4 into day 5 it would get lighter and lighter and usually would end within 4-5 days. Now the last 3 periods I've gotten have started out with slow spottings the first 2-3 days and then it will go BAM heavy flow for 2-3 days and then trickle on getting lighter and lighter for a week. I've also been noticing it's coming every 3 weeks instead of the normal 4. I am 21 1/2 years old and I know periods change but I've had my period since I was 10 years old. Is this normal?