Hi, and thank you for your question
It would be pretty unusual to develop symptoms of an STI that quickly after possibly being exposed. A very likely culprit of the soreness you felt might have been due to friction and not enough lubrication during insertive sex. Sometimes adding some extra lubricant can really help with comfort and with preventing damage to the skin such as small tears.
I don’t know what the discharge might have been, but it can be common to have some irritation to the urethra after having sex. During intercourse there are lots of microbes (non-harmful, normal healthy bacteria) that can be passed between partners. While these microbes don’t necessarily cause an infection they can get up into the urethra and cause some irritation. Staying well hydrated, urinating frequently (especially right after sex) and drinking cranberry juice or taking some cranberry supplements can help with this discomfort.
Finally, yes some STIs could be passed from your finger’s to a male partner’s genitals, if your fingers had been touching your own genitals first and there is an infection already present. The most common things would be infections passed through skin to skin contact such as HPV, herpes or even syphilis. So for example, if a person has an active herpes outbreak on their genitals and touches the sores, then immediately touches their partner’s genitals, it would be possible to pass that infection to the partner.
You might find our STIs at a Glance chart helpful as it gives a quick overview of how different infections are passed.
Let us know if this does not answer your question or if you have any more questions or concerns.