A one time exposure has a relatively low risk. So, you should keep that in mind. And you are undetectable. While there is theoretical risk of transmitting when undetectable, it doesn't mean it happens. This is a letter from the CDC that says it doesn't https://www.hivplusmag.com/undetectable/2017/9/27/breaking-cdc-officially-recognizes-undetectableuntransmittable-hiv-prevention I am sure your partner is going to be okay.
Risk is moderate given the fact that you were undetectable not too long back and also the exposure seemed to be brief. However, if we step aside from crunching probabilities to facing practical life, a risk is a risk, quantifying risk is ludicrous, it's basically like a Russian roulette scenario.
No one here can guess any thing, also you need to be responsible in the future given your serostatus, it is imperative that you draw your line.
Right course of action is to start PEP if your partner is still in the 72 hrs time frame, if not, wait for 28 days from the date of exposure and test with a p24 antigen / antibody combi test. For a conclusive result the same test needs to be repeated at 6 weeks. I am assuming that we are talking about a homosexual exposure in this context. If not, risk is low and chances of your partner contracting HIV from this event is very low. However, a test at 28 days is warranted for a conclusive.
Good luck.
For queries specific to yourself you would need to post in the "living with" of our website. This forum is only specific to HIV Prevention queries and information.