If you had sex with condom then its not possible that you get pregnant. As well if you have got period after that then its conform that you are not pregnant.
Here you need to check your
back pain started after abdominal problem or before it.
If back pain started before abdominal issue then back pain could be different and
abdominal pain could be different.
How ever the conformation will.be done by physical examination and few tests. so I suggest you to visit specialist and get your self examine, here for back pain if it is from long time with
radiating pain to legs doctor will ask you for MRI of back. As well for abdominal discomfort, heavy ness and
nausea you need to do
ultrasound abdomen.
Based on report you can discuss it with your specialist and plan for treatment.
In general here still till the time diagnosis is not conform I suggest to improve your food habits by having good amount of water and fruits in your diet, avoid spicy food. At the same time for your back use hot pack to relax the
soft tissue. This will also give you some more relief.
As per your explanation looks like you have got disturb gastro intestinal system and due to that you have
diarrhea and due to that more pressure on back muscles there is heavyness feeling too.
How ever the conformation will be only by physical examination and needed tests.
Take care