A lot of sexual arousal relies on relaxation (from my understanding this is especially true for women vs. men). You are in a safe place with no where you need to go with nothing specific you need to be doing or thinking about, you can relax and that is naturally arousing. This is totally normal and common.
Do you have a history of drug/alcohol use, or sexual abuse? I ask about the abuse because those type situations when you were a child could cause you to have a hyper sexual personality. If you do drugs or alcohol it could be a deficiency of serotonin or dopamine. When you abuse drugs or alcohol it causes your brain to over produce these chemicals. You get anxiety when you lay down to rest, and thus you're body is trying to find the easiest way to replace the lack of these chemicals in your brain.
Or, you know.. you could just be young and full of hormones. =3