Hello and welcome to HCM,
A delay of four days is not necessarily due to
pregnancy.
A delay of few days is acceptable in a normal menstrual cycle.
A negative pregnancy test rules out pregnancy.
Back pain and feeling tired can be due to a number of causes.
Tiredness can be due to
anemia, strenous activity, etc.
Back ache can be caused by a bad posture, trauma, as a feature of pre=menstrual syndrome, etc.
They are not only due to pregnancy.
Get a complete hemogram done and take rest.
If the back pain persists for long duration, you need to consult an orthopediacian.
Thanks and take care
Dr Shailja P Wahal