Cat cold: not getting better or worse for a long time.

[Nico]

Hello,
I adopted my 1-year-old female cat 3 weeks ago. At the time I adopted her she already had a cold and she came with 9 days worth of antibiotics (Clavaseptin). I was told she was under a lot of stress prior to adoption - she had recently had kittens, had been spayed, & moved around through a couple of foster homes. She's very petite, and I was told that her growth may have been stunted because she had kittens... but that might not be relevant to the issue at hand.

The antibiotics were helping, and she seemed to be almost cured. However, I ran out of them before she was cured entirely and I didn't want to go to the vet because I'm so broke from all these initial cat purchases, deposit to the landlord, and the adoption fee. That, and I just don't want to stress her out with more moving about (and I don't have a car!).

I've been giving her L-Lysine with her canned food from the very beginning. Last week I also bought echinacea to mix with her food, but I'm not sure if it's doing anything.
I've been treating her nose/eyes with a mild saline solution, which seems to help relieve the symptoms but it all just comes back the next day.
I also ordered Sinu-Rite and Respo-k online and am waiting for it in the mail.

She doesn't seem to be getting worse, although she's had 2 episodes of dry coughing/hacking in the past couple of days which hasn't happened before.

She has a voracious appetite! I might actually be overfeeding her, she looks like she's getting a little fat. She also has plenty of energy and demands a lot of play-time.

Should I be concerned?
Could the antibiotics have messed with her healing process because they didn't cure her completely from the start?
I want to know if I'm doing everything I can, besides taking her to the vet. I'm a student and can't afford vet bills, though at the same time I already love this cat and don't want to give her up... I feel a bit cheated, because I was repeatedly reassured by the shelter people that she'll get over the cold and everything will be dandy.