If a woman otherwise having regular menses misses her period, the dictum is first suspect
pregnancy, particularly if she is sexually active.
Preejaculating fluid is known to contain some sperms which are capable of fertilizing the ovum. Now it all depends when this incidence took place. Usually the egg is released in women about 14 days prior to the next expected menses. The ovum has a life of 24 hours while sperms have about 3-4 days. So you can calculate now whether your intercourse was in the fertile period.
With clothes on, the chances of pregnancy are very rare. However, you have noted your fluid on the vulva; so there are high chances that some part might have entered the vagina. It can cause pregnancy.
Now it is 7 days that her menses are delayed. So best option for you is to get the urine pregnancy test done. It will give you the answer to your dilemma. If it is still nonconclusive, you can go for ultrasonographic examination to visualize the pregnancy.
If all these tests show that she is not pregnant, then probably her menses are delayed due to
stress of becoming pregnant. The negative tests will take away her stress and she will get her menses.