The incident is believed to have happened sometime between 8pm and 9pm, in the area of Deepmore Road in Rugby.
Detective constable Thomas Willett from Rugby CID said: "This has, understandably, been a very traumatic ordeal for the victim and we have launched a thorough investigation to try and identify the offender.
"Police are supporting her and her family and enquiries continue.
"It is believed that an individual had ridden a bicycle through an alleyway on Deepmore Road around the stated times and may have witnessed the incident, and I appeal for them to come forward.
Anyone who witnessed or has information further to the incident is advised to contact police on 101, quoting incident number 104 of October 4, 2017.
Alternatively, independent charity Crimestoppers can be contacted anonymously - no personal details will be taken and information will not be traced or recorded and you will not go to court.
Call Crimestoppers on 0800 555 111 or get in touch via their website: crimestoppers-uk.org