Posture alone may not be the reason, do you have any other recent medical history?
Are you on any treatment for any other health problem?
If yes, kindly share details.
Next Steps
If you don't have any other recent health issues , visit your Physician who may refer you for tests (if required) to check if this has any internal underlying cause.
After this, consult a Physio for physical assessment and guidance regarding a proper exercise plan.
Health Tips
Basic deep breathing techniques and short walks may help initially.