What does it mean when your girlfriend used to always want sex but after a few weeks she doesn't want it anymore?

Just that, she does not want it right now. Our hormones can really do a roller coaster ride on our bodies. Just give her time:) Maybe that she is tired of trying to satisfy you while not getting any pleasure herself. Adolescent males are easily capable of having sex four or five times a day. Females in the same age bracket are far less interested in sex, and often can be satisfied with once or twice a week. Not until most women are in their mid-30's do they experience such strong desire as boys do when they are age 15 to 25. There are many possible reasons, all the way from she might have been rubbed raw from doing so often, to she has not been enjoying it at all. You might try asking her. But I think that there is a good chance that you are not meeting her needs in a sexual encounter. Boys can be ready for intercourse in a moment, but girls need to be stimulated by foreplay to be properly aroused and lubricating. Many young girls really prefer hugging and kissing to intercourse, and they always desire being close before and after intercourse. Ask your girlfriend if she is reaching orgasm from having intercourse with you. Don't feel bad if she says "No." Many girls have problems reaching orgasm from intercourse in the missionary position without additional stimulation. Ask her what she likes, have her tell you what she wants, have her show you how she wants you to touch her. Do you have oral sex before intercourse? This is an excellent way to bring a girl to orgasm, after which she will probably be more willing to satisfy your desires.