Welcome to the Forum.  I'll be glad to comment on your STI risk from the exposure you describe.  Receipt of oral sex is a very low risk sort of exposure.  There has never been a case of HIV proven to have been acquired from receipt of oral sex.  Chlamydial transmission in this way is also vanishingly rare.  Gonorrhea and non-chlamydial NGU are the two most common STIs acquired from receipt of oral sex.  When gonorrhea is acquired it is acquired is causes symptomatic penile discharge and burning on urination in about 95% or infected persons.  NGU is by definition, also symptomatic, causing burning on urination or a penile discharge.  If you do not have these symptoms you are most unlikely to have acquired an infection.  Syphilis is very rarely transmitted by oral sex and were it to occur, it would typically present as a sore at the site of contact (i.e. on your penis).  Herpes is likewise rare and most unusual if you already have HSV-1 the virus that causes cold sores and this too would most likely cause lesions at the site of contact. 

In that it has been a month since your contact, you really have very, very little to worry about.  There is no pressing need for testing of any sort however it you are nervous, a urine test for gonorrhea and chlamydia, as well as blood tests for syphilis and HIV would all be almost completely reliable at this time.

I hope this information is helpful.  EWH