Hi Praveen,
Obviously this is too strange. This will happen only when you are trying to increase the quantity of the material and the cost is exceeding the budget. There are some correction program available and you can also execute CJBW. But before you proceed to go for correction program or start executing CJBW and also since you are saying that you don't have a budget in your project, anyhow I will again suggest you to try following tricks to find out the possible root cause.
1. Check have you put any budget profile in project profile (OPSA). If yes, then remove it.
2. Check whether AVAC is activated or not.? If yes, then de-activate.
3. Check your cost determination log.
Regards,
Amit