I have started working out regularly for four weeks and have a sore rash under my breasts and between. could it be a sweat rash?

Likely: You should try to keep the area as clean and dry as possible. Using baby powder might help. For the rash you have now see your doctor who can order the proper cream to use and give you advise as above on how to prevent it from reccurring.

Heat rash or monilia: Most likely a heat rash or fungal infection. Best to be seen by your md before treating it yourself....