Thanks for your query.
I am quoting an abstract from a study published in 2011 (are you also mentioning the same study?)
"Tramadol (Ultram, Ultracet) is a centrally acting synthetic opioid with analgesic efficacy comparable to codeine. Antinociception is attributed to low but effective affinity for the mu-opioid receptor (μ), as well as reuptake inhibition of the monoamines norepinephrine (NE) and serotonin (5HT). Dual action antidepressants mirtazapine (Remeron), duloxetine (Cymbalta), and most notably venlafaxine (Effexor), which tramadol is closely related to in structure, also inhibit NE and 5HT reuptake. These medications are proven effective antidepressants and this shared monoaminergic action resulted in the research of tramadol as a potential treatment for depression"
Source: Examining the use of tramadol hydrochloride as an antidepressant
On the basis of mechanism of action, you can see that it might have some antidepressant action but I don't think that it is sufficient enough and probably that is why it is not a first line antidepressant.
Moreover, this drug can cause physical and/or psychological dependence also when taken for long duration. Combining it with SSRI antidepressants like Zoloft might increase risk of seizures in susceptible patients.
Now regarding your symptoms -
Is it possible for you to mention your symptoms again.
Currently you are taking which medicines?
Do you have any significant medical history?
Have you undergone any investigations recently or in past. ( specially Vitamin B12 or folate levels)
Please answer these questions and I will get back to you as soon as possible.