Why is my hair thinning months after bleaching it? Did I trigger TE or is it something worse? (Photo)

You need a formal evaluation to see what type of hair loss you may have. When you have hair loss, it's best to do a full hormonal workup for internal optimization, evaluate it with a dermatoscope, and consider scalp biopsies or scrapings to rule out any stress related, inflammatory, autoimmune or infectious causes of hair loss. In general, most people with early hair loss have telogen (stress related) and/or androgenetic (normal aging) and both can be treated with in office treatments like steroid injections, PRP/Acell, Exosomes and at home treatments like oral medications like Finasteride or Spironolactone, oral vitamins like Viviscal, topical exfoliating sprays like Restorsea, and weekly growth factors like anteagemd. See an expert for an evaluation. If hair loss is significant it may requires hair transplant (follicular unit translation).

Since you did not notice any immediate irritation or burning and think it is unlikely you have had any scalp or hair damage. I suspect that this has just brought some added face and attention to this are of the scalp.
Since you have dark hair and light skin this contrast will frequently give a false sense of thinning.
You certainly don't need scalp surgery at this time. You could consider a scalp camouflage like Toppik. Using a laser cap could be beneficial with out side effects. If it really is disturbing to you and you wanted to be a bit more aggressive, SMP (Scalp Micro Pigmentation) could be considered an is quite helpful in your situation.

It is very common when we first notice that we have less hair than we used to to look for a triggering event. That is because 95% of hair loss is caused by androgenetic alopecia which creeps up very very slowing. This is especially the case for those of us who are inflicted with the diffuse thinning pattern (rather than receding temples or crown donut).
With the diffuse thinning pattern what happens is over time one by one we thin and then lose just one hair from each affected follicle. On average we start with 5-7 hairs in each follicle. Over time that reduces the hairs in each follicle from say 5 to 4 then to 3 and then all of a sudden we have half as much hair as we used to have, but we haven't noticed because all our follicles still have hairs so we don't see where we lost it from.
What you are describing fits into what typically happens for diffuse thinning androgenetic alopecia.
When it comes to bleaching your hair, remember that the bleach is affecting only the hair outside of the scalp, it does not impact the hair follicle which is buried deep below the skin. Just like cutting your fingernails does not impact your fingernail growth - damage to hair outside the head does not impact the hair follicle underneath.
With regard to your hair loss - please make sure to get an accurate scientific diagnosis before starting any treatment. Each person's case and biochemistry is different and requires individualized treatment to be successful. Please find a precision medicine hair regrowth specialist who can order the Biochemical Report Card labs tests for you for accurate diagnisis.