Any ideas why I am so short of breath?

[Jellybeans9]

So this is going to be a long one, but I want to provide as much relateable information as possible.

For a bit of background: I'm a fairly healthy and fit person in my mid twenties. I try to eat well, I walk everywhere and I'm rarely ever ill. However, somehow, I managed to catch a nasty cough and cold that had been going around. I started to get better and then suddenly, ober the course of a day, got much worse. I was now experiencing a migraine-like headache, feverish chills and feeling lethargic. The next day I visited the doctor due to being unable to feeling breathless and my chest and back feeling tight, who told me it was just a viral infection, and to continue taking painkillers (paracetamol didn't seem to do anything for the headaches, so I was taking ibuprofen) over the next few days and drinking plenty of water.

Over the next few days, I was having coughing fits where I would struggle to get my breathing under control (I ended up drinking A LOT of water to help with this), however any mucous produced was clear. A few days later I woke up in excruciating pain. My left side and back (left side) felt like I had badly stretched the muscles. I'm pretty sure that's all it was since I had slept on the sofa (I couldn't sleep lying down) and had been coughing quite violently throughout the course of the night. I only had slight chest pain under my breast, which wasn't as painful and I attributed to the same thing. Also, whenever I coughed, a trapped nerve-like pain would shoot up the middle-ish of the left side of my back. It was so painful, I ended up taking co-codamol and using deep heat numerous times throughout the day. This actually helped to get rid of the pain and was practically gone the next day.

6 days after my original visit to the doctor, I went back as most of my symptoms had either gone or were getting better. After having spent 2 or 3 days on the sofa from feeling lethargic and generally just feeling ill,  I'd started to feel more myself. The only thing still lingering was the chesty cough (which was now not as frequent, but still productive - like a need to 'clear' your chest but nothing I can actually get rid of), and the struggling to breathe (like I had been exercising so couldn't take full breaths). The weird thing about my breathing by this point, was that if I were lying down or sat down for a fairly long period of time, I was fine, no coughing fits in an attempt to breathe properly.

Again I wad told my o2 levels were fine and my lung function mostly fine. I was prescribed antibiotics to clear up the lingering viral infection and an inhaler to help with the (almost) constant breathlessness. Im yet to see if these help. I was also told to have an asthma review with my GP as I had had asthma when I was younger, that I had grown out of (16 years since my last flare up).

The thing that is bothering me most is, I can't even walk 10 minutes down the road without feeling completely out of breath, like I have just ran a marathon or something. I'm really hoping the antibiotics and inhaler work (only had them a day so far) because it is really getting to me now. I am pretty much confined to the house until I can walk for more than 2 minutes without needing a break and I can't work at the minute either for the same reason. It's really getting me down and I just want my life back.