I will describe a brief encounter I had yesterday evening which got me worried.

Specifically, I met a transgender woman younger than me - 25 years old (born as a male, still having male genitalia) and shared some intimacy as follows:
- she performed unprotected oral sex on me twice, where I ejaculated once

- I performed oral on her anus

- at some stage she was "on top" of me and while masturbating me, she rubbed my erect penis towards her anus. She poked it on her anus, applied gentle pressure, but I recall no penetration happened. If any penetration occurred, it was very superficial (I would have realised my penis getting into her, I believe) and lasted for no more than 5 seconds. This entire thing lasted for max 10-20 seconds as I didn't want to engage in unprotected anal sex with her and I stopped it.

I got concerned afterwards, mostly about HIV. I asked her about her situation and she said she got tested in May/June, after which she got multiple partners. She stated that she had protected sex with all of them except for 2 whom she claims she knows and trusts.
I was wondering if PEP was required in my case and hence went to the emergency dep in a local hospital (it was nightime), where they told me my exposure was low risk and I didn't need any PEP. They also said I don't need to get tested.

Today I rang a local STI clinic, where I described the same situation and they reiterated my exposure was low-risk, no PEP needed, and they booked me for a complete STI screening in 5-6 weeks.

I have been searching on this site for similar situations - which I could find, but wanted another opinion as this matter got me quite concerned and worried (I tend to get anxious about such things).
Thanks in advance. 

1.  Most transgender women do not have HIV and your partner not only told you she had tested but when she did.  Most people tell the truth.

2.  If she had untreated HIV, there is no known risk of acquiring HIV from receipt of oral sex and the risk of getting HIV from analingus is also close to zero- conservative estimates put the risk, in the unlikely circumstance that she had HIV at less than 1 in 10,000 encounters.

3.  If your penis entered her anus, which sounds unlikely, and she had HIV (also unlikely) the risk of infection from this circumstance would still be less than, on average, one infection per 1000 encounters.

I would not worry.  I hope this information will help you to not worry either.  EWH

- I am pretty sure no penetration occurred. There was just a bit of rubbing of my penis head against her anus, and an "apposition". I can exclude any proper penetration inside her anus. And the duration of this "rubbing" or "frottage" as we want to call it, lasted for max 10-20 seconds. I guess in this case the 1/1000 chance you mention about insertive penetrative anal sex reduces?

- given the above, would you agree with the ER doctor who told me I wouldn't need to test? The STI clinic doctor said the STI screening scheduled is for "peace of mind", mostly.