Thank you for asking the query.
This particular spotting you have described in your consult is a known side effect of depot preparations ( injectable contraceptives). I hope you know that the depot injections are female hormones which will be slowly released over the days. While this depot preparations can prevent unwanted pregnancies certain minor unwanted side effects can also occur. And the most common one among them is irregular bleeding. These hormones can interfere with your natural hormones by creating a negative feedback system. And if there is inadequate hormone support to the uterus it can shed the endometrium which can be manifested as bleeding/spotting.

Cramps, sourness and bloating can also occur with depot preparations. But I understand that your specific concern is whether this is some 'more significantly' serious health issue. Well, it does not look like one for me. But its always better to be sure. So I recommend you to visit an OBGY and consider the following :

1. An ultrasound scan of the pelvis is a safe and easy way to look for anything abnormal. Discuss with your Doctor whether a USG can be done.

2. A Pap smear test

3. Early pap smear screening . This needs an risk assessment by your Doctor.

One rare possibility is that during the second injection an adequate dose was not delivered to you. You can discuss this with your Doctor. 

There is no specific treatment needed for the bleeding as of now. The best approach would be to give some time so that it settles by itself. 

Please feel free to ask followup questions via this email itself. Thank you.