Things do not necessarily look good for you. Chastity was an important aspect of virtue for a young English noblewoman in the later Middle Ages, as it was even as far down the social scale as the urban middle and servant classes. There are numerous examples of men being prosecuted for seriously slandering young women's sexual behavior or for being caught in a premarital relationship, because of the damage done to their reputations. When Robert Chew deflowered Isabella Alan, both in the upper ranks of the peasantry in the mid-fifteenth century, he paid her the great sum of twenty marks in recompense - equivalent to a year's income for a gentleman, or the cost of a nice house and furnishings - because the hit to her reputation and future marriageability was so severe. If the father (or another man) married you after the birth, your baby would still be a bastard and not eligible to inherit his property according to Common Law, though the church would have allowed them to be legitimized. If the father with the king, though, he could grant a child lands to derive rent (as a dowry if a girl, with a title if a boy), and arrange for a good marriage. In non-royal families, there was still usually an attempt to set up an illegitimate son with some sort of land to live on and an illegitimate daughter with some form of dowry. But the vast majority of these examples involve women ranked below the aristocracy - members of the peasantry or bourgeois classes, or servants who had children with their employers. The amount of money and land that was involved with aristocratic marriages and inheritances made it extremely unappealing for a nobleman to wed a woman who had already fallen short of the one-sided, misogynistic standards of sexual continence: who was to say that she wouldn't cuckold him and bear children that weren't his? As a result, noble families were serious about keeping their daughters from having procreative sex before marriage.

However, Europe saw changing standards of the concept of legitimacy and illegitimacy between the ninth and thirteenth centuries (the late Early and early High Middle Ages), so it really depends where exactly you're situated in the period. In many cases, maternal and paternal lineage mattered more than the parents having a religiously sanctioned marriage in terms of whether or not children could inherit property. The Carolingian and Ottonian royal dynasties cared more about whether or not a child's mother was of high status than whether a religious wedding ceremony had taken place between its parents or whether such a marriage had been annulled; William the Conqueror was recognized as his father's heir despite his parents being in no kind of formal relationship, and his mother's low birth seems to have been a bigger deal to his contemporaries than that. All of these situations required that the father take some responsibility for his children, though, and be of pretty high status himself - it wouldn't really work going the other way.